That has never been true.

You can even "buy" open source software on Steam that is available free from the home website.

You're referring to the case with the developers of Lugaru, and they've been doing a great job of painting their fuckup as Valve barring platform competition.

No, they don't want you selling Steam keys for cheap on other platforms.

Valve actually doesn't earn anything from steam key sales. They don't take the 30% cut on those, only on sales made in the Steam store. The only requirement Valve puts up is that you don't start selling these steam keys (which are provided for free) cheaper than you can purchase the game in the Steam store.

I don't see that hindering competition at all. It basically allows other marketplaces to sell Steam games, enabling competition in the first place.