If they're ready to attack they wouldn't waste resources trying to stop a rebellion, they would tell Skyrim that they're going to attack. That in it's self tells me the Empire is still a long way off doing anything, after 30 years.
They're not ready this instant. Why would you tell Skyrim you're going to attack, Skyrim itself isn't fighting the Empire. Do you mean tell the Stormcloaks that the Empire is going to attack the Aldmeri Dominion?
Besides the obvious fact the Stormcloaks are in arms against the Emperor and the Legion. The Empire 'wasting' resources trying to stop a rebellion tells me they're like any ruling body. Is it normal for Empires and Governments to not spend resources trying to end rebellions that are against them? The Legion considers the Stormcloaks to be a threat to the Empire, that Ulfric is a thug and power hungry usurper.
If the Nords supporting Ulfric couldn't figure it out from all the standard Legions in High Rock and Skyrim (and the Legions of Cyrodiil) marching south to the Dominion's border in recent years. Nearly the entire Imperial army sitting on the Thalmor's doorstep? Clear sign of aggression.
If someone places nearly their entire army on your border, how would you translate that action? "Just going out for a stroll"
Also the Legion actually makes many remarks towards fighting the Aldmeri Dominion.
"Unless the Empire stands together, the Thalmor will destroy us all." - Hadvar
"But Ulfric and his "Stormcloaks" are deluding themselves. If there's any hope of a long term victory against the Dominion, it's in the Empire. The rebels are only inflaming the tension and weakening the Empire by distracting it from its ultimate aim." - Legate Rikke
"The ordinary citizen will be happy to get back to life as normal, to have their families return home. And they should enjoy it while they can... I suspect all of Tamriel will again be called to arms in the not too distant future." - Legate Rikke
"The fiercest of the remaining rebels will continue to harass us, but by and large, the people here desire peace. What I'm not so sure about is the peace we've made with the Thalmor. But we'll keep that between the two of us, alright?" - General Tullius
Most of the Legion is tied down on the border with the Aldmeri Dominion. The Emperor can't afford to risk weakening Cyrodiil's defenses. From the Imperial City, our war here is just a sideshow. An interlude before the main event against the Thalmor resumes. - General Tullius
Then you also have the comments from every Legion Legate that make mention to something along the lines of
"Skyrim's days are darkening, soon the Legion will be called into service like never before"
Though of course one can ignore all that and pretend the Empire and Thalmor are best friends. You don't need to even look that hard to find mentions of another war coming. It isn't like the Empire keeps it a huge secret, the Thalmor themselves even make mentions to it.