This was news to me. Not that “some have suggested…” sounds particularly convincing.

“The origins of such are marker are not 100% confirmed, however some have suggested that the gesture dates back to 1927 when Cardiff City won the FA Cup final against Arsenal and opted to celebrate by changing their square flags to triangles in order to rub their glory in the faces of rivals, Swansea City.”

https://talksport.com/football/fa-cu...a-everton/amp/