Thats not true first of all.

You are pay protected even if you never knew the trip existed, if it should have been assigned to you. No acknowledgement required, nor does it have to be placed on your schedule. These things, when caught by DALPA, pay out all the time to pilots who didn't even know about it.

Secondly, WRT ARCOS, the discussion was it being cancelled during the assignment window. By definition there is no such thing as acknowledgement in that situation. All someone can do it say yes I'd like to do it, however until the window closes that may not be relevant at all. Likewise, someone who didn't yet put them selves in line for that trip when the window was cancelled does not lose any assignment rights because there is no such thing as first come first served before the window ends.