Is Stormheim reason enough to declare war? Yes, to an extent. But is it a justifiable one based on what we know? No, it isn’t. If anything, if she is using it as an excuse it just sort of digs the whole of being manipulative and evil even deeper.
A lot of this is covered in the rogue campaign, which in my opinion was one of the best chains Blizzard has done. There are heavy SPOILERS for it in the rest of of my post, so keep that in mind.
The reignition of hostility with the Horde started at the Broken Shore, not Stormheim, when it was made to appear that the Horde had abandoned the Alliance to their fate at the hands of the demons. In fact, the entire thing was a setup, false intel given to the Horde and the Alliance both to lure the bulk of both their forces and the Argents into a trap. This was done by way of a false Mathias Shaw, as well as infiltrating demons such as those revealed by the demon hunters when they show up.
Now, the Broken Shore was the straw that broke the camel’s back, and was the entire impetus for Genn going after Sylvanas. Had that not happened, the attack wouldn’t have happened either. And both sides knew this, and know this, because both have access to the members of the Uncrowned who discovered it. Stormheim happened because Genn wanted revenge for what he perceived as a betrayal, a betrayal that was only able to be perceived because of the machinations of the Legion.
Moreover, in the patch immediately afterwards, when attacking Suramar, the blood elves and night elves and high elves are working side by side. Now, the Alliance forces are just that, part of an Alliance, and more or less can do what they want. But Silvermoon is beholden to the Warchief, and was dancing to Sylvanas’ tune in large part even before she took up the role. They would not be there without her approval, and this means she’d more or less let the Stormheim thing rest.
Of course, she does bring it up later on, as a way to prove her point that the Alliance might attack them later. But in reality her entire reasoning is speaking to a history that belongs to the Horde, not the Alliance. She knows that any clashes with the Alliance in Legion were the result of Legion meddling, ultimately, and so her using Stormheim as reason to start the war is inherently self-serving. The Horde at large doesn’t know what she was up to in Stormheim, of course they’re going to believe her.
From the in-story point of view? Yes, it can be used as justification in the eyes of the average Hordie, if not justification for the Burning (though they’re apparently ok with that). From a meta perspective taking into account all we know? It’s not, and Sylvanas also knows it’s not. Saurfang should have known it too, the moment the plans for the War of Thorns started, but he’s senile.