When you say knock down, you have to specify. Hard and Soft Knock downs are two different things, are you sure that people can't tech roll out of orbital strike? Are you sure that Death Stroke can't his Sword Flip invincibility while at fullscreen to escape because you can't punish him for it from Fullscreen, unless you have Orbs set up.
Lex's armor isn't that good as you're making it seem to be, Most multi-hitting projectiles can beat it out, Frost's Daggers, and All guns are examples.
How can Lex even knock Deathstroke down from fullscreen?
Edit: Orbital Strike starts up in 74 frames.. That should be plenty of time to roll out, but I'll test it tomorrow.