I think the point made was though that you cannot swap U2VS air groups to Il-2s or anything else. You can of course create other tactical bomber airgroups and swap them with other models (not U2-VS). And I assume disband U2-VS if you reach the number of airgroups limit.
Yes, if a squadron is listed as an "NBAP", then their aircraft cannot be swapped for IL-2's (or any other types, I believe). But if a squadron is a "BAP" or has no designation (just the number), it can have U2VS as its plane type. And you have tons of U2VS's. The limiting factor, as you suggest, is the number of air groups you can have. I used to make tons of NBAP squadrons, figuring that they were a training ground for pilots who could then fly more useful planes, but have subsequently realized that this is a trap and those squadrons can only ever be disbanded (and don't do much damage anyway). So now, I build regular tactical bombing squadrons, set them on night missions when their experience levels are low and German fighters are plentiful, and, if IL-2's are short, swap them out for the endless supply of U2's. I should make that clearer in the doc, thanks.