No, they haven't. I don't know where you even get this impression. TF2 isn't optimised for lower spec machines. It works on systems with sub-par GPUs, but the Source engine is CPU bound, not GPU bound. The early versions of TF2 were fairly optimised and could run on most machines. The laptop I had when I first played it was definitely inferior(on paper anyway) to the one i had now, and it ran much better. Now it's a complete mess. It gets slower every update.
I very much doubt it's an external factor outside of their control. Fact is there's not been a single optimisation related update(bar one regarding the netcode, which made no difference to me, it's still ****).
The fact that Left 4 Dead 2 runs slightly better despite the hordes of Zombies shows it obviously is something within their control. Though honestly that could be improved too.
Threads like these are very common:
Note this quote -
There was a really long thread on steampowered but I can't seem to find it.