Sure, you have to protect the teleporter area, but the whole point of the teleporter itself is area denial.
You're putting down the teleporter down in an area to plop out your teammates so that they can deny the area and lock it down. Just like a Bastion or Toblerone. Lucio, or any other support character isn't really doing anything similar regarding denying the enemy an area. The only remotely similar thing is Mercy's ult, which is still more of a sustain over a denial.
Again, I think this is exactly the same as Lucio.
How does Symmetra's ult deny an area? By making it more likely that teammates are in that area, by teleporting them to it if they die.
How does Lucio's aura deny an area? By making it more likely that teammates are in that area, by running them faster to that area and making them less likely to die while in that area.
I think this is not a sensible definition of area denial.