I'm amused to note that this probably applies to Yahoo Groups as well, since it uses the yahoogroups.com domain, not a yahoo.com domain.

Looking at the headers of a recent message from a Yahoo user over Yahoo groups it seems like it would be the case:

  dkim=pass [email protected];
  dmarc=pass (p=REJECT dis=NONE) header.from=yahoo.com

Of course, anyone who is using Yahoo Groups regularly has probably noticed that even with last year's redesign it's not getting much attention.