I'm also surprised that none of the "Peter Sellers tape" was used.
Save the mono mix included on the Blu-ray disc, everything in this set is newly mixed/remixed. The original album is a remix, the Esher demos are freshly mixed from the multi-tracks, and the three discs of session material are all fresh mixes.
While they certainly could have gone back to the original EMI alternate mixes that were dubbed to the "Sellers Tape", I think they were going for the most interesting and different studio outtakes. The Sellers tape has mostly just minor mix/editing variances compared to the commercial mono mix. Those extra measures near the end of "Sexy Sadie" are probably the *most* noteworthy thing on the tape.