Valve gives you free steam keys for your game on request, which you can sell off steam, without paying Valve a cut. This has a specific rule that disallows selling those keys for a lower price. However, not sure if it’s this case, there was an email submitted as evidence from a Valve employee telling a game developer that selling their game for less in general would be undercutting steam, and something they wouldn’t want. If the email is real and not a misinterpretation, Valve indeed was/is pressuring developers to not sell games cheaper elsewhere.

Also, sales and giveaways are exempt from the steam key price parity rule, which I would assume epic’s free games would fall under, if you applied the rule to that despite not involving steam keys.