Hmm. If you couldn't, other devs might have trouble finding a defnitive answer too and just play it safe.
Also, they might have changed policy over the years, like restricting it to only affect keys being sold. They were sued for this parity thing after all, so it at least once was a rule. But I am not sure about the current situation.
https://www.techpowerup.com/294750/valve-antitrust-class-action-lawsuit-allowed-...
The Key Price Parity Provision is a policy that prohibits game developers from pricing their games cheaper on competing storefronts
If I understood correctly, the sueing party was not even allowed to sell their own game on their own website without a steam key.