Not even making that bit up, it's established SovCit lore that pops up now and again. I'm a boat, you're a boat, your mom is a barge lovely lady (and also a boat).
yes, they believe it's one of the "in plain sight" pieces of evidence - that when you're born, you have to have a berth (maritime) certificate that makes you like, property of the state or something because you're recorded as "docking" with the country (which is actually a corporation that wants to own you and use your cache of money they hide from you that you receive at birth), and "they" get away with the ruse because us normal rubes just never think to ask if they said "berth" (maritime) or "birth" (reproduction). not even joking lmao
it is a funny pun, too, but they really think birth certificates are, in fact, secretly "berth" certificates for them in maritime trade law, which they believe is the only real law, in which we are all like.. sovereign vessels that have been duped into signing away our independence.