Okay, to reverse the question then - have you ever seen anything that supports there being something contractual to say thar non–Steam copies can’t be sold at lower prices? Like, the terms you mention above? I’ve read the public docs, and can find nothing.
I can think of multiple times when, e.g. Ubisoft games, Rockstar games, have been sold on EGS or their own launcher for far cheaper than the version on Steam - so we’re both supported by anecdata, here.
Which part of the industry are you looking, if you don’t mind revealing?