Someone correct my if I am wrong, but if you are applying to become a merit source, do you have to show 10 under merited posts/threads from your local sub ONLY? A merit source application is supposed to show that you know what a merit-worthy post is supposed to look like. And theymos doesn't select merit sources to distribute merits in specific local boards only. You do it everywhere. With that in mind, find any 10 posts that you believe are merit-worthy but they haven't received enough merits. I feel like that should be enough.
I don't think that there are any rules from which boards those 10 posts should be, but it kinda makes sense that if you are applying for a local board merit source (and some people do that), it kinda makes sense to include mainly posts from that local board, therefore showing that local board is indeed under merited and that it needs more merit sources.
I also remember members making merit source application for other English boards and including only posts from that specific board, like for example
NotATether merit source application for Development & Technical Discussion board.
As you said, theymos wasn't specific from which boards those posts should be so you could just share 10 quality under merited posts and that should be enough, but I guess that people think this way they have better chances of getting it approved.