True, but the Guardian of Forever's planet appears to have some shielding against timeline alterations, so the landing party continue to exist even after the Enterprise vanishes. Same thing with Spock continuing to exist and Kirk's mind remaining unaffected in Yesteryear.
Burnham's timesuit having similar shielding might work as an explanation, which could expand to the Discovery since it follows right after her, but then the Enterprise, along with L'Rell and pals, should have all found themselves in a new timeline right after Discovery left where the battle they just won never took place.
I should probably mention I don't mean this as a criticism of the show, getting mired down in this stuff is pretty much my favourite part of time travel stories.