How could bands like COC and DRI who were late comers in the 80s movement, and influenced by pre-existing thrash bands, who were originally playing hardcore(not thrash at first, that came later) after the term thrash was being used to describe bands like Overkill and Slayer, be the only ones called thrash...
All those are good examples thrash/crossover. As in Hardcore that was crossing over into thrash, a genre of metal that already existed. Most of those bands didn't even get started till the 83 and 84(by then Overkill, Metallica, Exodus, Slayer, Metal Church, Bathory, etc had albums or demos out), and most had a purely hardcore era before listening to Metallica, Exodus and Slayer and the rest and thinking "ya know, musically this stuff is related to what we play anyway, and has more balls then pure hardcore, lets fucking THRASH IT OUT"
Very simply: your facts are wrong.
DRIs first works were 1982, and COC was of similar vintage. This was all inspired by Discharge in the same year, which is why 1983-1985 was such a potent time: hardcore finally produced something which fused easily with metal.
Back then, punk wasn't as metal as it is now, so to punk ears DRI sounded like metal
Further, "thrash" as a term was used before the music came around, to describe the culture from which this music originated. (You can find lyrical hints to that culture on the first DRI album.)
At the time, most people who were the types to later get into underground metal REFUSED to refer to speed metal as thrash for that reason. The magazines got ahold of it, and thus Metallica et al became called "thrash." Eventually, this morphed into "thrash metal" for the keyword happy.
However, originally thrash and speed metal were quite different, often antagonistic, genres.