True, but so can nitrogen, propane, neon, carbon dioxide, hydrogen and flourine (although you'd need to be pretty unlucky to survive long enough breathing flourine to asphyxiate).

I know argon is denser than air at STP, and so tends to accumulate in enclosed spaces --- but propane is even denser. (Leading cause of catastrophic accident in yachts: gas explosion. It's not like you can have a hole in the bottom to let the gas out in the event of a leak.) And any plant which deals with liquid gasses is going to prioritise ventilation anyway, which should easily deal with any gas buildup.

The parent sounded like there was something specific to do with argon which made the regulators tetchy, and I wouldn't have thought that asphyxiation risk would be enough. Am I wrong?

Propane has a very strong odor added to it (by the propane manufacturers), which means it'll be pretty damn obvious the moment you enter a propane-rich environment.