Some recipes have a clear definition: spaghetti alla carbonara does not contain halloumi. Risotto is made with short-grain rice. Souffle involves whisking egg whites. Etc etc.
Others are adaptable (in fact, risotto fits here too: you could make a risotto with many types of meat, fish, veg etc, as long as the rice and technique is right). Trifle fits into this category. It's a blueprint, but it's highly adaptable - different fruit, flavours, etc.
When people point out the former, they're snobs who want to stop the poor from eating. But apparently there is only one trifle? Yawn.
PS she regularly turns into the Guardian's How To Eat column. The Guardian generally has good food pages, but How To Eat is lame clickbait designed to get people shouting "JAM FIRST YOU PRICK" at each other in comments. It's so, so, so inane.