Kael'thas is pragmatic politician so he would choose their survival and he knows yesterdays enemy can be tomorrows ally and horde wasn't even close as sunwell pretty easily stopped them and its different horde as the clans which made it up were totally different. Also worshipping no working with yeah as he had belfs would have no other choice and nelfs invaded belf landed at the start of tbc sent agents there and killed all who found out it was a quest in ghostlands.
Its not fic as it came from blizzard and as such is canon and placing kael on tbc lor'themar position alliance member sent military troops to you territory and sylvanas offers to help you reclaim you lands and kael as pragmatic politician would see that he needs allies to survive because of his numbers are pretty low and alliance have again proven hostile and sylvanas offers a way to the horde which shares the name of the former enemy but even he knew orcs were demon manipulated because of antodidases research on orchiss fel withdrawal. As such anything dark mirror states is 100% more canon than anything either of us would state.
Kael murdered most of the watchers pretty brutally to save illidan and Jaina had already proven his genocidel tendecies as purge of dalaran specifically towards elves and Tyrande would treat later on kael as she treats all elves who drained fel magic to survive in short pretty badly. As Tyrande would despise kael for what he did to draenai in outland and what he did to gain exodar.
So Tyrande sending troops in to ghostlands attacking elves, supporting faction which tried have him and his people killed and alliance haven't even tried to make reparation for it and for supporting a man like that it would extremely unlikely Tyrande and kaels relations were in that deep it was a mission in wc3 and the whole time she talked about Tyrande as priestess which is keeping distance emotionally more than calling anyone by their surname. Jaina is a different thing but Jaina chose a genocidal maniac still rather than him and later oon slaughtered sunreavers justified maybe in a way but still kael would see it genocidal and see it similiar to whar Garithos did.
Also when was Thralls horde genocidal? most likely it would have Garroshes horde which might have driven Kael out of horde and also it was horde heroes according to chronicles 3 which saved belfs from the threat of zul'jin and kael would be extremely thankful towards the horde.
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also this "In addition to the list of affiliations above, Kael'thas was also loosely (if not earnestly) allied with Thrall's Horde for a brief period, if only as Quel'Thalas's sovereign. Kael was kept up to date on the state of negotiations by Rommath, and it is strongly implied in Blood of the Highborne that Kael himself was the source of information mentioned in one of Lor'themar's letters that confirmed the presence of uncorrupted orcs still on Outland; Kael's regent used this as a bargaining chip with the Horde." as such Kael knew about blood elves joining the horde because of Rommath and we have no knowledge of him opposing it as Rommath was one of his closest advisors.