I have seen no comments (not to mention photos) giving any indication that the "bikini" BT was supposedly wearing was referring to both top and bottoms. The word "bikini" could mean both pieces, actual swimwear. Or it might mean bikini top, without specifying what she wore below -- shorts or whatever.
Ditto with any mention of a black bra, I do not recall any mention of underwear bottoms -- no specifics were given at all, IIRC, about what she wore on her lower body, other than the mention of the presence of socks and shoes.
While sun-lovers might take walks in a skimpy top, it really seems beyond reasonable to wear only swimsuit bottoms or underwear -- one wouldn't even be able to stop and sit on a boulder for a rest -- with thin material it would be too hot to make contact!
I have to think that she would have chosen at least shorts. If she is out there with only swimsuit or underwear bottoms, I have to think that either she expected to go only an extremely short distance, or that she was not choosing her clothing voluntarily...
IMO