I don't agree. The wording goes like this:
This states the better progression is taken, not the better BAB. Taking only the best BAB of all the classes we have ever gestalted with (yeah, I verbed that noun; I'll do it again, too) effectively erases advancement in all but one class. Rather, at each level, we should progress as the better of the two classes we are currently gestalting with.
Fractional bonuses make figuring this out easy. I'll wager we were supposed to use fractional bonuses (and it just so happens that that particular rule is presented on p. 73, right smack dab in the middle of the gestalt character section) for all base bonus progressions. By that reasoning, we have 1 level at which the Ftr progression (+1 BAB) is the better progression, and 4 levels where the Sor/Wiz progression (+.5 BAB) is our only option. That gives us a total BAB of +3. All 5 levels have contributed to the character's total BAB, whereas in your logic the first level Ftr is thrown away entirely (or rather, retroactively changed to using the Wiz's BAB instead of the Ftr's).
Just my $0.02.