Never said a Will can never blitz. You said that, not me. I said a Jack linebacker will never blitz from the WILL linebacker position. As in Gass will never play the “WILL role” See what I mean?
It’s more likely they walk a SAM or WILL linebacker UP to the los to blitz to take advantage of their speed. In other words, in blitzing situations, a WILL or a SAM can act more of a Jack, but a Jack will never act like a WILL. They’re never going to back a Jack up to the traditional WILL position to blitz from there, thus completely negating his speed.
The only time he goes backward will be in disguise pass coverage situations, not a blitzing situation (as you correctly mentioned). Jacks/LEO’s either have their hand down or are standing up near the los in blitz packages.
A square is a rectangle, but not all rectangles are squares. Hope this helps.