This is actually the flimsiest of arguments when it comes to Gojoseon. The fact of the matter is, there are no records of the Gojoseon language. The first records of Koreanic are tentatively from the Three Kingdoms period. The exact dating & placement is still contested among linguists, but it is in the vicinity of the Korean peninsula. Whether Gojoseon's elite language was an earlier prototype of this language vs. a different language that was displaced by Koreanic is an open problem. It's not though language displacement was rare in this area of the world - simply look at the case of Japonic, which used to be located in the Korean peninsula before it was displaced there by Koreanic, and in its turn displaced the languages of the Jomon Japanese when it migrated to Japan.
Not to mention, language, much less linguistic families (separate languages that are not mutually comprehensible) and ethnic identity is not identical, which has long been established in anthropology.
Modern definition of language often requires an acceptance of its identity by a common community of speakers. Since there was no such thing in the 1st century BC, to label them all as one language, even if they are of the same linguistic family is misleading.