I'm just personally not seeing what you're seeing with Harmon. His help defense in the post and quick hands have definitely been good, and his recovery defense has been solid, but he is getting beat off the dribble continuously. And just to make sure my eyes aren't deceiving me, here's the defensive analytics for our players thus far adjusting for opponent (Note: The more negative number the better as it refers to the hundredths of a point prevented per opponent possession they account for when on the court):
From Hoop-Explorer
Dain: -7.3
Guerrier: -5.1
TSJ: -4.0
Ty: -3.9
CoHawk: -2.3
Domask: -2.3
Amani: -2.2
Moretti: -2.1
DGL: -1.9
Goode: -1.5
Harmon: -0.7
Now granted, this is imperfect as it doesn't account for the level of the individual opponent on the court at the same time as them, i.e. a starter vs. a bench player, however, it is safe to say that Amani, Moretti, DGL, and Justin all have somewhat artificially inflated stats if anything as they've spent a larger percentage of their minutes against backups than the starters.
So at this time, thru 6 games, Justin has had the worst individual defensive share on the team of the non-walkons. And that really matches the eye test as I'd only put him ahead of DGL at this time. I'm not trying to bash him either, I'm simply saying that Justin really needs to show more than he has if he is going to play a role as a defensive stopper on this team. In fact, analytically, the only category that he's shown to be a distictly plus player is his offensive rebounding share. Everything else for the most part is subpar.
So to summarize, this is nothing similar to Ty where analytically he looks extremely good as well as passes the eye test. This is more a case of a player not passing the eye test for the most part and also thus far being subpar analytically.