All that really means is flamers have become ineffective if you use the trainer (learned this myself)
That being said... the trade off for being able to fire without heat concerns, really is more powerful than enemy mechs not overheating.
My plans to roast pilots in their Mechs is kaput, but considering the other advantages I'm not concerned.
Heat is a global value, so that's probably why ANY edit to heat applies globally, instead of just to your units.
PRO TIP: Trainer Heat option resolves BEFORE heat is applied to your mech, so alpha strike past heat warning threshold without fear. (have not tested this on a Full Bar yet tho, just built a tri-PPC and large laser boat tho, so we'll see if i instapop on an alpha or not.)