There’s a lot I don’t like about how a lot of card magic is presented. A lot of them are “magiciany” things that we think everyone knows. Let’s start with the fact that there’s a chunk of people that don’t know what the spade and club suits are called. We assume they know that, they don’t.
The one that gets me is that magicians expect people to know what the “mate” of a card is and they use the word “mate” which really is an industry term. It’s not a hard concept to grasp, but when we use industry terms it doesn’t really help. Recently I watched someone do a card trick, they showed the seven of spades and they asked, “what’s the mate to the seven of spades?” The person didn’t know what they were talking about. It would be easier to say, “that’s a black 7, so we’re looking for the other black 7, which would be the seven of clubs…” There’s really nothing gained by asking them what the mate is, and not telling them.
I think “mate cards” are the card magic version of calling scarves “silks”. It still works to convey the idea, but it doesn’t work really well.