ONLYOFFICE accuses "Euro-Office" maker Nextcloud and IONOS of License Violation
(www.onlyoffice.com)
Yes, a creator can create a closed source copy (independent of the GPL'd work), assuming they have full ownership or permission from all contributors
It's not fully academic only because it's common for companies to develop a GPL version and offer commercial licenses where the corporate customer is exempt
But none of this affect the users of the GPL version, so most people don't care