It has nothing specifically to do with steampipe. Many SourceMod functions just use direct filesystem calls from the game's base instead of looking through all of the valve filesystem search paths (including the ones in
gameinfo.txt which was augmented in TF2's steam pipe update).
This is or was mostly because that interface had a tendency to change in breaking ways on some games. It's possibly a bit more stable now, but would require a new build (not just gamedata) anytime it breaks with the current setup.