Nevermind I did some more testing and looks like super grabs work differently than regular grabs. I guess if you manually time your SA3 to have invulnerability during the active frames of Ed's super, then for some reason you can grab him during active frames.

Checked in training mode and figured it out. Manon only has 6 invulnerability frames, while Ed has 9. So by the time Manon is grabbing, Ed is still invulnerable. Her grab only has two active frames as well, so she couldn't possibly have grabbed him no matter what here.