You're right, but it's influenced by the configuration of "Time Limit on Editing of Posts". If you want the members to be able to delete their own threads forever, you must configure this value to 0 and with that you are allowing everyone on your board to edit their posts forever. This is something no one would want.
It's quite a pity "Time Limit on Editing of Posts" can't be set based on usergroup.
Maybe this one's for you:
https://vborg.vbsupport.ru/showthread.php?t=117483
Ingress