That might be your definition but it isn’t the official definition. The ‘wonder’ part is about how successful that one song is, which Whiter Shade of Pale was. Because for that matter, nobody would be a one-hit wonder, since musicians do a lot more behind the scenes than we realise, like in the Trevor Horn case. But one-hit wonders are still a thing and Procul Harum are one of them. I realise that they may have a lot of fans but that doesn’t take away that moniker.