that is a tattoo (which I didn't know Billie had) but I don't think its Billie cause Billie is busty AF, like her tits are easily double that woman's, probably even triple the size of that woman's (at least), also I don't really pay attention to things outside of her tits and face but yeah i don't think her hair is that long, just compare the hair length in that pic to two pictures of her in public that occurred either side of it (or at least a pic/s that came before that if there aren't any pics of her in public yet since when that pool pic is from)
Also picture is to zoomed out to be certain if it is Billie based on the tattoo since you can't clearly see what it is, just think about the number of tattooed women, and then the number of women with tattoos in similar positions you could probably easily find at least 10 women on this site alone with tattoos in that spot
also could look at the two women with the supposed Billie (well the one you can kind of see in the pool taking a picture) and look to see if that may be someone Billie is know to be friends with or that she could possible have a meeting like this with, although she strikes me as a woman who would only get nude like that near close friends