I've never heard of bloom nutes reducing stretch. I've always found it mainly due to genetics and partly due to environment.
Do you mean cutting grow nutes now would reduce stretch? Or is there something in the bloom nutes that would do it?
From my understanding and this is also MY OPINION.... but Bloom nutes will slow stretch when introduced to soon for autos. With autos the best way to get Transition nutes is waiting for the stretch to almost stop before switching to bloom nutes. When you first start seeing signs of preflowers you want to keep running veg nutes for a week or 2 and then slowly introduce your bloom. This isn’t the same for photoperiods since your choosing when to flip and change nutes.
Hope this helps I can find a good write up for you if you lenintersted! Let me know