Apologies if it's my ignorance, didn't grow up playing the game here, but this puzzles me.

I get a safety.
https://nfl.myfootballnow.com/watch/13161#2314966

Next play, rather than kicking away to me as was previously done, Bills now try an onside kick - I wasn't aware that could be done after a safety?

https://nfl.myfootballnow.com/watch/13161#2314967

Onside kick can be attempted during any free kick in the old kickoff rules

That I did not know.

Glad it's legal, helped me win ;)