Iron Treads: Why, Game Freak? WHYYYYYYY?!!

yes because mirror herbs are for teaching egg moves to pokemon without breeding. it's an intended feature

Is Steel Roller even an egg move at all in SV?

It didn't work when I tries that.

The Mirror Herb technique usually works, but it's not going to be useful here. I think what @CyberBearIV is trying to say is that there's no amount of Mirror Herb usage that can allow any newly-hatched Pokemon to inherit Steel Roller from Iron Treads, and that the only way to get Steel Roller in Scarlet is to trade an Iron Treads from a Violet savefile to your Scarlet savefile.