You're right. Soft/Medium stategy is enough for 25% races. As someone who does 25% all the time I can say it is the case, although there are some tracks where tyre degradation seems to be bigger than others.
The only time I consider using a Hard tyre at 25% is if for some reason I decide to start on the Mediums and I have a crash early on that forces me to pit in the first laps. Using the soft tyre for the rest of the race could be a bit of a stretch so I sometimes chose to go with the Hard tyre but yes, in general, they are obsolete for 25% races. The AI just doesn't seem to be coded to "think" that way. I think they are coded the same way for all the race lengths.