You're the one making a positive claim... The burden of proof is on you. Jesus Christ...
Except what you're deliberately ignoring here is that Stormheim was maybe mentioned once, but it was mentioned to convince Saurfang and justify the war to him. And would you look at that, Sylvanas is planning to conquer Stormwind only after Stormheim has happened, while previously she was so interested in a war with the Alliance and conquering Stormheim she sat out almost the entirety of the previous faction war after she consolidated her power in 4.0. It's almost as if Sylvanas' entire modus operandi was a desire to live forever and removing all threats to that goal. With Alliance proving itself to be a threat with Stormheim. And if Sylvanas has a casus belli with Silithus, her war is a reaction by default, because that's the very point of such casus belli. Her just making plans prior to that is really immaterial.
Except Sylvanas dwarfed Garrosh's achievements at the very start of the war, with War of Thorns succeeding where Garrosh has failed for three years. Hell, most of actual "good showing" during the previous faction war was Sylvanas conquering multiple Alliance zones and beating Seventh Legion while Garrosh fucked around with Magnataurs, Molten Giants and whatnot with little to show for it. And if not for Jaina ex machina the entire Alliance army would be done for at Undercity. Given how the Alliance army breached into the city near the end of the fight and Sylvanas Blighted it, a ton of them still likely have perished in the end, with Jaina being too busy elsewhere to freeze the Blight again. And disproportionate response is pretty much how all major powers wage war. It's part of their military PR to dishearten other potential enemies.
Something something burden of proof being a thing.
Which is precisely why Genn says multiple times he has no clue at all what Sylvanas is after even after the attack at the start of Stormheim. Or makes no reference to the book when he's pondering about the topic while he's making the remarks about being clueless.