I’ve understood this to be common practice for years.
You can find instances of people discussing it and explaining why on this forum and elsewhere.
e.g. Lower trainer difficulty for racing? - #10 by Mark_Murawski
most racers probably set it to about 20-25% from what I have heard. I keeps your effort more consistent overall, but it also allows you to push much more on downhills when you otherwise might spin out too much. I usually ride at 40-50% so I can keep it in the big ring and don’t have to shift as much.
Or What’s the Best Zwift Trainer Difficulty Setting for Zwifters? | Zwift Insider
So how do I handle my personal Trainer Difficulty setting? It generally stays around 30%, except when I’m doing a flat TT and remember to bump it up to 70-100%.
But it’s by no means universal.