Nah, evolutions have happened in Gyms before, it's simply a style the writers like to do and think will suit well for the situation.
p.s. if your judging this from the BW070 preview, we don't know if Tranquil will evolve after the battle yet, I assume it'll be against Swanna but you can't confirm it until it airs.
Who evolved in a gym battle, pre-Isshu, if I may ask? Since I can't remember any... Because none did ever before.
It's one thing to do something unprecedented, but to do it in half the gyms in a region is too much.
And the original post is also wrong, not all pokemon evolved against TR and their mecha's. A lot did, but especially in DP, and AG even a bit, Ash pokemon evolved against rivals:
AG:
- Treecko evolved in a rivalrous battle against a Loudred which also evolved.
- Grovyle evolved in a battle for a Meganium, in which a Tropius was his rival for her love.
- Glalie evolved against TR iirc.
- Swellow evolved in the Pokéringer?
- Donphan evolved against TR.
DP:
- Starly evolved against TR
- Staraptor against Paul
- Grotle against Paul
- Gligar evolved to save Ash from falling
- Chimchar against Paul
- Monferno against TR-mecha falling ontop of Electabuzz
- Cyndaquill against TR-mecha
So roughly only half of those were TR induced. So we already saw a decline in their need to be present for an evolution to happen.