True enough, that definition would indeed allow the term "race" to be applied to the range of groups I mentioned, Catholics, Protestants, gays, Republicans, Democrats. Even wider - to cover "women", "men", "children", or morris dancers.

The Concise Oxford English Dictionary's equivalent sub-definition is clearer in indicating the kind of context in which this meaning can be properly used: class of person etc with some common feature (eg race of poets, of dandies, etc.)

But I think that moving on from there to use the word racism to cover prejudice against any and every "class of person with some common feature" is extending it a bit too far. And I doubt if there's a dictionary definition of that word which would recognise that use.

People do use the term racism in relation to prejudice against religious groups - for example, Catholics and Protestants in the Northern Irish context, or Muslims or Jews more generally. I'd see them as using it in an analogistic way. Making a point of enclosing the word in inverted commas is a way of representing that.