OSM doesn't exactly have a threshold for how much population a place needs to be classified as a city. Wyoming has no cities that come close to passing 100k in population, but has 10 places tagged as cities in OSM. Colorado itself has a notable example—Grand Junction—which has 65.5k pop. as of 2020 but has been tagged as a city on OSM since 2010. 

Additionally, multiple examples of legal towns tagged as cities and legal cities tagged as town. A place's legal status is arbitrary in the grand scheme of things as a map user is likely not concerned with the legal status of a place but rather its size and regional importance.

I found one mention of Parker on the OSM Wiki, but this doesn't seem to revolve around place classification and only use the word "town" as Castle rock is legally a home rule town while Parker is a municipality that self-declares itself a town.

"Colorado has two basic divisions for incorporated municipalities, towns and cities, though Colorado law makes relatively few distinctions between the two. "In general, cities are more populous than towns, although the towns of Castle Rock and Parker have more than 45,000 residents each, while the city of Black Hawk has fewer than 120 residents."

This excerpt, at least from my perspective, would actually be a good argument for why legal status is arbitrary, considering a legally-defined city like Black Hawk can have a tiny population while places that are legally municipalities, towns, villages, hamlets, census-designated places, unincorporated communities, etc. can have varying populations and in many instances be larger in population and more important to its region than what places local governments have decided to classify as a "city".