as others have said, it's designed for anyone who has a period, so transmen who have periods are part of the target group.
I mean it's a bit of a random advertising idea, because:
a) if the idea is to sell as many items of your product as possible, based on the most recent census figures transmen are going to be less than 0.2% of the target audience, and it's likely that (given this thread) a higher proportion than that might be offput by it
b) the whole point of including models in photos is surely to give an indication of what the product would look like on a real person, and the vast majority of people buying period pants won't have a body like that model....but then I suppose neither do most women have bodies like most models that are used.
so it might be a bit of a strange choice, but not an inappropriate one.