Yeah, that's correct. On its own, it's just a head unless it's either iconic in design itself (Batman's mask, Vader's helmet) or a 1:1 recreation of a piece of artwork or photograph. The smartest play, given that they've already drawn a lot of attention to themselves with this, is to put it with a completely different figure or in some kind of decent-size accessory set.
If they just release the two heads entirely on their own right after releasing their not-Sonja figure, while it's not infringement, it literally sends the message that they just don't care and are going to do whatever they want ("here's the Red Sonja body, and here's the Red Sonja heads") and I think/hope they're smart enough to know that's going to cause a lot of friction with people they've already upset and who, I imagine, they don't want to have bad relations with.
Smart play is 'well, we made the head so we'll stick it on a totally different figure and if the FANS put it on another figure in their homes, that's their business.' I know it seems like a small distinction, but that really does -feel- different than 'you made us take the heads out, so we put them in a package by themselves for the express purpose of the fans buying those heads on their own to use with that previous figure.'
By that logic, it's not IP infringement if you make and sell a He-Man figure as long as you sell each part of his body in a different package.
Which is actually TRUE. But it's still going to have Mattel's lawyers spending a lot of time watching you very carefully and looking for an excuse to make your life miserable. Why go there?