Makes me wonder if any/all existing UE3 games will work on linux now
It's not that simple. UnrealEngine 3 from 2012 isn't UnrealEngine from 2011 isn't UnrealEngine from 2010 and so on - developers take a snapshot of the master UnrealEngine code, and hack on it from there. There's a fair amount of porting work involved in doing a major update to the engine, depending on how customized a specific game's copy of UE3 is.
Not to mention the common use of (platform-specific) middleware in addition to UE3, such as Havok or Wwise. Even if a game were using exactly the right version of UE3 to be easily recompiled using this WebGL port (unlikely) or the Dungeon Defenders Linux port (equally unlikely), then using third party middleware with no Linux port, like Wwise, kills it.
This port going into the mainline UE3 source tree will help a lot (the Dungeon Defenders Linux port is not in mainline UE3, it's available only from Trendy Entertainment), but only for new games without fancy features that necessitate use of middleware.