There are no minor differences in appearance and language between Stormwind Humans and Kul Tirans, what is the difference between Jaina and Anduin? And their culture is extremely similar to Gilneans due to Kul Tiras being a Gilnean colony. All of those groups are still different despite next to no differences, they just established their nationality, were pretty loud about how they wished to be addressed and that was the end of the story.
High Elves and Blood Elves are pretty loud about how they wish to be addressed to. There are NPCs, there are quests, there are novels, etc.
If them telling you how they prefer to be addressed is not enough and the requirement for being a different group is "different appearance, language and culture", well, remember High Elves had blue eyes, Green elves had green eyes (and plenty of them still do), High Elves still speak Thalassian while due to Horde influence Blood Elves had to learn other languages, it would be impossible for them to interact with the Horde otherwise. And i'm pretty sure being part of the savage Horde vs civilized Alliance leads to big cultural differences, Blood Elves have been obsessed with the color red since Frozen Throne, had a history of interactions with fel + killing of magical creatures to survive, tortured and drained a Naaru to be able to become Paladins, participated in events like the Bombing of Theramore, tried to betray their faction during Mist of Pandaria, stood by the Horde as their warchief tried to enslave the Valkyr, stood by the Horde while their warchief genocided the Night Elves, etc. while High Elves stayed in the alliance or were in other parts of the world like in Outland, refused to use fel, participated in the Purge of Dalaran, etc.
There are differences in appearance, culture and language, and funny enough after all those events, their differences are bigger than Night Elves and Nightborne, specially when you can make Night Elves that look more Nightborne than their actual current model allows them to.
What Blizzard employees have to say on their twitter accounts is not part of the lore. Specially considering their history of saying stupid stuff like "If you want pale blonde elves the Horde is waiting for you" and then since they kept losing players they just gave Void Elves options to look identical to the High Elves Ion was so adamant against letting people play in the Alliance.
There is a discussion to be had, the discussion just doesn't have to include any fallacy of appealing to someone's authority. Instead our discussion should include scenarios in game in which a High Elf and Blood Elf outright tell the player "yup we're the same there's no difference between us" which is not a thing that ever happened in the game and even members of other races, like Varian, Jaina, Rhonin, etc referred to them as different groups.
Legion even had this entire cutscene in which Elisande considers High Elves and Blood Elves different groups.
https://www.youtube.com/watch?v=fn4M1q-a38E