It could be sage mode, she said 'it's almost done' why would she need prep time to store up and release the seal now, it should have already been stored up if that was the case.
If it isn't sage mode it might be something else resembling Tsunades technique without the drawback, or maybe healing on a level Tsunade could not reference to when she said rasen shuriken damages on a level that not even herself could heal.
SM wouldn't be so much out of the question, people might mention jiraya and Tsunade but Sakura is the only one of them who has been hyped for her chakara control over and over again. We don't even know the method slugs use to gather Nature chakara so we will just have to wait and see.