Of all the things it could have been, how the heck can you say that so absolutely? If Twitter was otherwise working, I agree that it was likely an isolated Spaces issue. As was said, Spaces is fairly new and does not seem to scale well. 1 million people trying to join the Space at the same time is a plausible cause.
Again, if Twitter was working fine for those not attempting to join, it's unlikely that it was DDOS. Again, assuming that first part is true. This is a technical opinion. Let it shake out.