Roadrunner is not an obscure song, Phish has played it before. Nobody who knows that song could play something that similar to it without realizing what they were doing. That riff is iconic. Key and tempo should not matter when considering a tease. The whole idea is that they are referencing one song in the context of another song, so of course they are going to stay in the tempo and key of the song they are actually performing.
Also key and tempo are not copyrighted components of a song (they aren't integral to what constitutes a certain song). I could play Chalkdust in any key at any tempo and it would still be Chalkdust.
On the subject of teases, that Mumford & Son's tease is so clearly intentional. You can hear Mike stumble on to something similar and then find the right notes. Mike teases all the time without getting credit for it.