That just seems like a consequence of English having the convention that compound words are written with spaces between them. There is no meaningful difference

That just seems like a consequence of English having the convention that compound words are written with spaces between them. There is no meaningful difference (apart from the language) between the English "vaccination passport" and the Dutch "vaccinatiepaspoort". For spoken languages, spelling is secondary to speech, so this convention shouldn't matter on whether or not something is a word. There is surprisingly no all-encompassing definition for "word" in linguistics, but given the fact that you cannot split up "vaccination passport" without either part losing its meaning, it makes sense to award it an entry in a dictionary either way.