Yeah, basically after they'd worked in the studio for about the "normal" amount of time, Rabin took all the tapes (actually digital multitracks) back to his place and played with them for hours and hours. 13,000 hours is just Anderson using hyperbole, but by all accounts, it was a lot of time and he tweaked everything. Individual notes were adjusted, entire sections were reworked, drums sounds were removed and replaced by triggered sounds. It is widely believed that even though Tony Kaye is credited as the keyboard player, there's nothing left on the album that he actually played; his parts were all re-recorded by Rabin. Some bass parts, too.
Rabin knew that this would almost certainly be his last Yes album, and he wanted it to be perfect. And it does sound fantastic, I must admit. Somehow, I guess I prefer a bit less polish. To me it sounds over-produced, tweaked and corrected until there's no life left in the performances, because they're not even musical performances any more, but digital reproductions and simulations of musical performances. It sounds great, though.