Not possible to get a late EX raid invitation?

In the past, I've read people talking about doing a normal raid at a gym after they found out it had an EX raid scheduled, then receiving their own invitation once they did. However, I've also seen multiple sources say that once an EX raid is pending, no normal raids will happen there until it's done, so that first idea is impossible. So which is it? The latter, right? If you weren't in the initial batch of invitations, too late?

My area has a Mewtwo scheduled for November 18th, but apparently only two people actually received invitations for it. Unless some more of us are able to get delayed invitations, they won't even be able to win.