Not that I defend all of Valve’s business practices, but last I heard, you’re only prevented from sell games cheaper on other platforms if the thing you’re selling is Steam keys.
AFAIK, this restriction doesn’t apply if you’re distributing via a different market place.
If that’s true, that seems fair since you’re relying on Valve’s infrastructure to support the sale.
Is that a thing anymore?
Humble Bundle 100% does both, sells games cheaper and redeemable through Steam keys, and has been doing so for many years.
Humble bundle might have a deal with them, given the charitable nature and also how long-running it is.