They're probably open and felt the need to mention it specifically. I know people like that as well.
Your phrasing reminded me of what casual mentions of a same-sex partners used to elicit, when heteronormativity was more prevalent.
If I'd said the same thing I would've tacked on (poly) or (open) for clarity because it's wholly unconventional and anybody who's been in that kinda relationship is constantly aware of it.