I know next to nothing about coding, but manually opening communities to DoS seems like trying to fill up someone's email inbox by sending someone thousands of emails with one word in it.
I don't know if it is possible, but I suspect there is an automated way to do it. Imagine you are using a client like thunder or liftoff. The "Create community" button is linked to an access point to the Lemmy API. If you know how to do it, you can DoS the instance. At the very least, you annoy the rest of the users.