Again, per the link prior: this standard predates even Office itself. They could have not followed the standard, but then they would have angered users for not following standards used elsewhere in PC applications at the time.
F2 has long been "Rename Object" and F3 has long been "Find" (though Excel in this case doesn't use it for that, but that's another matter). Did IBM do enough user studies at the time? I'm not sure IBM could have found a user study to predict the order of those choices being irritating to Excel users two and half decades later.
Even then, too, the IBM CUA I pointed to was partly synthetic, pulling in commonalities from apps at the time of its writing. Given F2 is nearly universal as "Rename Object" [1], I've got a feeling it predates the IBM CUA by quite a bit in some heavily used applications, but I'm not a historian.
Office never existed in a vacuum, and the function keys have had common meanings for decades. Microsoft used a mix of the IBM CUA guidelines and presumably what they were seeing from the competition (Lotus 1-2-3 and VisiCalc especially for Excel early in its history). If they put the useful function of F2 somewhere else, they would have broken the muscle memory of so many other PC users, even early in Excel's history. If they moved it today there might be riots, some rioting even by the same people removing F1 keys from keyboards.
[1] You can see it on this chart and compare it with most of its neighbors: https://en.wikipedia.org/wiki/Table_of_keyboard_shortcuts