When you think about it, the TF2 universe probably COULDN'T be canon to the Half-Life universe
A big plot point for HL2 is that humans have discovered localized teleportation, and the Combine wants access to it.
Them not accessing it from Aperture makes sense, since it's a large hostile facility located underground.
But if TF2 existed in the same universe, then teleportation would've already been invented and understood by humans (since it was invented by the Australians, among another technological leaps)
Anyway, just a thought I had as I was trying to consider the TF2 universe and how it'd fit into Half-Life. Any additional thoughts or counterpoints are encouraged :^)