While I won't pretend I'm super knowledgable about Primaris Space Marines, thinking they're one of the dumber ideas Games Workshop had, I believe they are the product of
Belisarius Cawl who is the most non-Ad Mech like Adeptus Mechanicus
Yes, we do know it works, Xenology literally has someone dissect an Ork and discover that weed killer affects them. As for DAoT we don't know a lot about that era.
Now if you have a quote of them trying weed killer and it not being effective on Orks that would be different.
It's an odd oversight but if canon demands they haven't done so yet I imagine that's due to plot armor so the Orks aren't wiped out. Obviously that shouldn't be a concern for this scenario.
I'm going to need evidence to support your assertion. Just like I'd like evidence for Ork tech requiring reality warping powers to work.
Wait, human mother and an alien father? Joining a predominantly human led organization and wearing blue? Hi Spock!