They don’t really though. They’re talking about selling steam keys in a different platform, not selling the game on a different platform (like Epic Games for instance). You can sell the game for cheaper on Epic or GOG if you want to.
When new video game stores were opening that charged much lower commissions than Valve, I decided that I would provide my game “Overgrowth” at a lower price to take advantage of the lower commission rates. I intended to write a blog post about the results. But when I asked Valve about this plan, they replied that they would remove Overgrowth from Steam if I allowed it to be sold at a lower price anywhere, even from my own website without Steam keys and without Steam’s DRM.
They don’t really though. They’re talking about selling steam keys in a different platform, not selling the game on a different platform (like Epic Games for instance). You can sell the game for cheaper on Epic or GOG if you want to.
From the source cited by the article.