I still maintain, just like Stone Fox, that they are mistaken. It is likely not intentional, since the legal stuff can be frightening and complicated, but noone that says this ever managed to quote any part of the steam ToS, Partern Agreement or Distribution Agreement that requires this, not even anonymously. Also it would not be legal. All "Most Favoured Nation" agreements are usually quickly put down in court (see e.g. the practice booking.com had in the past. At least for the EU they have been forbidden to do this).
Part of this misunderstanding is the issue with the EPIC / Sweeny frivolous lawsuit that put out lies / deceptive statements to this effect (without even knowing the details just stipulating that there might be some NDAed agreements or "secret contracts" that require this) and David Rosen from Wolfire, who blantantly lied about this. It is really annoying how that shit sticks after it has been thrown and devs do things based on these lies that have no merrit. See for example this link for some more details and links to articles:
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