>>714145653From this clip, it looks like you may unironically have been a second too late, but the way the Dark Dimension Cape works is that if your teammates press any button besides jump, they'll lose the invul.
It's one of those abilities where you genuinely just have to time it to the exact moment an ult will hit because your teammates aren't gonna know you're going for it in the middle of a firefight.
Also, be warned, using this puts not just you but anyone else caught in it in a sort of "Out of Play" state when it comes to contesting point, so if you use it with a second of time left on the OT clock, it will count as you or anyone caught in it 'leaving' point, i.e., losing. So be mindful of when and where you use it in overtime.