Yu Lin Bloodhawk had initially considered using the promise he requested from Lothar and the Kingdom of Stormwind's Little Prince to deal with the coming undead invasion. But the moment he thought of Archimonde, who would follow Arthas's Undead Army, he immediately dismissed the idea.
Archimonde was a violent maniac whose combat strength was simply off the charts. Yu Lin Bloodhawk absolutely did not want him coming to Quel'Thalas, so he chose the approach he had previously believed they should take when Arthas's Undead Army invaded.
As for why he had asked Lothar and the Little Prince for that promise, the real reason was simply that, although establishing the Kingdom of Stormwind Restoration Corps would solve some problems on the Seaside Plain in The Hinterlands, Yu Lin Bloodhawk was unhappy about working for nothing while also handing Lothar and the Little Prince such an enormous favor. He kept feeling as though he had gotten the short end of the stick.
So Yu Lin Bloodhawk had gone to Lothar, struck a deal on behalf of Hinterlands, and picked up a little extra benefit along the way. He did not care what anyone else said, nor did he know how he would use that promise. He simply felt that only by getting something in return could he feel comfortable, rather than being plagued by the feeling that he had lost out.
In truth, Yu Lin Bloodhawk himself did not attach much importance to the promise, and he had never given any serious thought to how he would use it. Personally, he did not believe the high elves' catastrophe could be resolved through an alliance with someone else. He considered placing his race's hope of survival in another's hands an exceedingly foolish, dangerous, and unreliable idea.
Only by saving themselves could the high