Thomas J. Beaver writes ...
As usual, there's a short version, and a long version.
Short version -- that's John. He perceives that he's being challenged, so he comes back with the most cutting thing he can think of (in this case, verbally slapping George in the face, that they’ll get the very guy George looks up to …). For John, it does't mean he really means it - what he means, is "Oh yeah? Take THIS!" SLAP!
The longer version is a story ...
On January 10, 1969, the 7th day of the making and filming of Let It Be, they spent much of the morning working on the instrumental track for "Get Back" (which Paul hadn't finished the lyrics to yet ... and one take even had John singing the lead vocal!). There was tension (of course, there was inherent tension going back to the "White Album sessions over a year earlier) -- this was Paul's song after all, and, as usual, he had things planned out as to what he wanted it to sound like, how he wanted George and others to play ... so they moved to working on "Two Of Us."
In the Beatles' Anthology, George said about the tension, “Paul wanted nobody to play on his songs until he decided how it should go. For me it was like: ‘What am I doing here? This is painful!'”
And here's the infamous clip from the movie -- the row between Paul and George -- taken during this morning session (see the 34 second mark):
George: “I’ll play whatever you want me to play, or I won’t play at all if you don’t want me to play. Whatever it is that pleases you, I’ll do it.”
Now irritated as well that the row with Paul was filmed for the documentary, at lunch George surprisingly was a no-show ... then he walked in and said, “See you ’round the clubs." He left.
John, as was his wont, immediately struck back verbally, saying, “Let’s get in Eric. He’s just as good and not such a headache.”
Paul and Ringo would not be drawn in ... and 10 days later, George returned.
In the Beatles Anthology, George explained what happened from his point of view. "They were filming us having a row. It never came to blows, but I thought, ‘What’s the point of this? I’m quite capable of being relatively happy on my own and I’m not able to be happy in this situation. I’m getting out of here.’
And, more from George: "Everybody had gone through that (with being frustrated). Ringo had left at one point. I know John wanted out. It was a very, very difficult, stressful time, and being filmed having a row as well was terrible. I got up and I thought, ‘I’m not doing this any more. I’m out of here.’ So I got my guitar and went home and that afternoon wrote "Wah-Wah”."
As far as John goes ... there was more than just being hurtful towards George ... he had gotten to know Clapton some. He had played with Clapton the year before — and not only on the White Album's "As My Guitar Gently Weeps" sessions.
There was also the temporary Super Group: Dirty Mac. It was on the Rolling Stones' "Rock and Roll Circus" TV show just shortly before, in December 1968. Clapton on lead guitar, Lennon at the mic and guitar as well, Keith Richards on bass, the great Mitch Mitchell on drums.
(for much more on Dirty Mac, see: Thomas J. Beaver's answer to Who was in the supergroup Dirty Mac?)
So ... John did know Clapton.
As for Eric Clapton however ... in 1969 he'd had his fill of band infighting with Cream - which is why he broke the band up. He went looking for a 'family atmosphere' ... he thought he might have it with his buddy Stevie Winwood, but at the first session of Blind Faith, there was Ginger Baker who Clapton was sick of - so that was that. He’d also shown up at the door of The Band’s Deep Pink house in Upstate NY, hoping to joinn them … that was a no-go. Then he hoped Delany Bramlett with his wife and band, "Bonnie and Delany and Friends" might be it ... and he joined them (I saw them ... Clapton on rhythm guitar, not opening his mouth) but then he saw Delany's controlling side, and the Bramletts' increasing physical-fighting with each other, and that was THAT.
So ... the last thing Clapton was looking for in 1969 was to join a fractious, bickering, splintering group called The Beatles — particularly when that’d mean displacing his friend George Harrison.
In Martin Scorsese’s Living in the Material World documentary, Clapton spoke extensively of his friendship with George and his interactions with the other Beatles. Regarding the White Album sessions he’d taken part in, Clapton had nothing but praise for the Fab Four.
Prior to Clapton’s solo on “While My Guitar Gently Weeps,” no rocker had gone into a Beatles session and played lead on a record. Nonetheless, he recorded the solo and it turned out great.
Then at one point in the film, an interviewer asks Clapton if he’d heard the story about John saying, "Bring in Eric" after George walked out on the group in January 1969. “Yeah,” Clapton replies. Then Clapton gets asked if he ever considered what it would be like being in The Beatles. Clapton responds by breaking into an extended laugh.
Then he replied, “Yeah … the pros and cons of being in a band like that were massively extreme. There were times when it was like the closest-knit family you’ve ever seen in your life.” But the flip-side was what things had gotten to by 1969: “The cruelty and the viciousness was unparalleled."
Anyway, it all came to nothing.