Its not true. You're allowed to sell for lower elsewhere, but you can't sell steam keys for a lower price than steam store. So if you create a version of your game that works without steam you can sell that for a lower price.

In the class action case[0], which was allowed to go forward by the court last year, it is claimed that Valve told someone:

"This includes communications from Valve that “‘the price on Steam [must be] competitive with where it’s being sold elsewhere’” and that Valve “‘wouldn’t be OK with selling games on Steam if they are available at better prices on other stores, even if they didn’t use Steam keys.’” Dkt. No. 343 ¶ 158, 160 (quoting emails produced at VALVE_ANT_0598921, 0605087). "

(This is a new case, not the 2021 suit, which was rejected by the court, then amended and refiled, later with an additional plaintiff added)

[0]https://storage.courtlistener.com/recap/gov.uscourts.wawd.29...