"Place stirrups onto your mount, allowing you to interact while mounted in the Broken Isles"
Considering Argus technially is not the Broken Isles leads me to believe it won't, but I don't have the PTR installed to test it so I figured I'd see if anyone who does have this item could give it a go.
Went ahead and tested them, and sorry to report they did not work on Argus on the PTR. Also spotted someone else with the stirrups buff, and that player also had to dismount to mine. I did not try herbing with it, but I'm assuming the same is true for herbs.
With just a few days til the patch releases, I'm doubting this will change but who knows.