I think the second "gotcha" is that even at 83 microns, that particle size is likely unsafe. That's about the size of gypsum dust used in drywall and you can't cut or sand or do anything without PPE for long without serious health consequences. Make it any smaller and it'll be well into the asbestos size range, which would probably be carcinogenic based on the mechanical damage it causes alone (a lot of nanoparticles are toxic for this reason). That's safe in a controlled industrial environment with everyone using fitted respirators but spread out on fields exposed to wind, near residential areas? Dumping that much fine dust everywhere would probably create a similar hazard to the dried out beds of the Great Salt Lake or the Salton Sea or Blackrock Desert* except everywhere there is farmland.

It'd have to be combined with a binder to form much larger particle sizes so that the natural weathering safely exposes the olivine slowly over decades. I suspect that once one does the math on the whole lifecycle, it's net carbon positive only after decades if it is to be done safely.

* People who have been to Burning Man can testify to just how much trouble dust that small causes, not to mention the frequent dust storms with zero visibility.