Is there anything to suggest he's even a mediocre route runner though?
The reality is unless any of us has been watching a ton of Washington tape without telling the others, none of us knows. It's not like there's a ton of tape and data to make it obvious, there's just very sparse usage on the field, both in college and in the NFL. I imagine anyone going to Steelers training has a far, far better idea - but that's none of us. None of us know how he looks there, or how hard the Steelers have tried to develop him there.
What I do know is there's not a whole lot of proof for his athletic gifts extending to change of direction, which is the athletic gift I'd tie most to route running. He didn't do the three cone drill at the combine which is probably the most telling for CoD, and that was when he was at 264, not the 300-odd he's meant to be.
And that in general, the best route runners in the NFL are frequently not all that athletic, and that it's far more of a technical art than one of raw athleticism anyway.
He's also playing for one of the most TE-centric OCs in the league. Smith's offences were in the top 5 for the percentage of passes thrown at TEs for 4 years prior to this year, where they were merely 8th. If Arthur Smith doesn't want to throw at a TE, I think it's really likely there's a good reason for it.