Honestly I don't find it too hard to believe. Just because we fight a common enemy doesn't mean any and all animosity suddenly disappears. Being able to work together doesn't mean they suddenly stop hating one another.
In addition, the alliance has some plausible excuses for going to war whether the Horde attack first or not. They still want Lordaeran, Genn still wants revenge for his son, etc.
Only reason I could see Horde striking first is because Genn struck first in stormheim.
The timeline and other various unknowns still need to be known before we can give it a fair judgement.
In either case, I personally enjoy the faction war, so i look forward to fighting the Alliance!