Wikipedia can be accurate, if based on credible sources. But there does seem to be a problem here.
According to the footnotes on Wikipedia’s “Birthday Cake” article, the Protestant claim came from an hobbyist website called New England Recipes. Specifically, it refers to an article titled Birthday Cakes: History & Recipes
However, if you visit that source, you will see that there is no claim of birthday cakes being a tradition of Protestant countries. That claim seems to have been an invention of whoever wrote the Wikipedia article. The claim should probably be edited out, if anyone here has an account to modify Wikipedia entries