From the opinion piece:
Last year, I pointed out how many big publishers came crawlin’ back to Steam after trying their own things: EA, Activision, Microsoft. This year, for the first time ever, two Blizzard games released on Steam: Overwatch and Diablo 4.
I think it only prohibits them from selling steam keys for cheaper elsewhere. It serves to protect steam from bad actor publishers that would try to cut steam out by selling keys on their own website, not paying the 30% platform fee while still using steam’s infrastructure to deliver the game to players. Source
It’s amazing that steam offers this functionality at all, not even mentioning they don’t charge anything for generating keys.
Reddit is not a source, but the source linked in that post isn’t really clear.
However, in this Ars Technica article they state ‘Sources close to Valve suggested to Ars that this “parity” rule only applies to the “free” Steam keys publishers can sell on other storefronts and not to Steam-free versions of those games sold on competing platforms. Valve hasn’t responded to a request for comment on this story.’