Ay, rule can be traced back to this forum post
https://www.warzone.com/Forum/218347-clan-league-updateI generally agree that the rule can be improved and it will for next CL. I think it's fair to say that when written the rule was not considering something similar to today's situation. Changing an old and established rule suddenly right before the signups end is just not the right way to alter rules. That being said, I do encourage any clan who wish to use this rule for CL 11 to go well above the minimum requirement.
Does that mean that whoever signs up until 13th NWO will also plays in B now to fill this group?
It's a bit hard to speculate Clans for B when A is not settled yet and signups are open. Thru it is possible that there will be extra promotions if needed.
Someone asked about Discord too. I don't have the time to manage/moderate one more Discord server, however I am sure the owner of the CL 10 discord is happy to pass it on if anyone wants to moderate it and go and asks for it.