No, it's only true if you're selling steam keys for cheaper on other sites. Devs can generate steam keys for free and have other sites sell them, but they're not allowed to list them for cheaper than what's on steam. Steam makes $0 off these steam keys and it still costs them money (bandwidth and such) so it's a more than fair deal asking to not list them for cheaper.

If you're using a completely different distribution system/client like itch.io or the epic store, you're allowed to list them for cheaper.