There is a singer in Oxford who will tell you to shut up if you try to accompany the song.

Trying to accompany an accompanied song is risky and potentially rude. Apart from the problem of finding the key (which could be unplayable without retuning) there might be variations in the tune, the verse structure or tempo - many old source singers did this. I know several songs where the structure is not uniform.

There are also times when someone starts a tune and nobody joins in, not because they can't but because the general feeling is that the tune and player deserves to be listened to.

As for the incident mentioned, the singer was treated apallingly. Session etiquette can be a hard thing to define, but these "musicians" don't have a clue.