Riding SS/FG can definitely help expand your comfort zone as far as having to spin faster or push harder for a given stretch of the road.

Even spending a lot of time on my older 2x5 and 1x6 friction-shift bikes, makes it feel easier and I go moderately faster on the same routes on my modern 9/10-sp brifter bikes, because I don’t need to push as close to my performance limits for as long as on a bike with more limited gearing.

Hitting and holding those peak efforts is what does make you “stronger” but also tires you out, and the longer you hold those peak loads, the longer you need to recover.