> You may as well say that UPPAbaby strollers are their own market category because UPPAbaby stroller accessories don't work on other strollers.

I wouldn't say UPPAbaby strollers are their own market, but I would say UPPAbaby stroller accessories are their own market, at least if they have and act on a legal way to prevent all others from making and selling accessories that interact with their strollers.