This match is a good example of how even good players in Zero-K are bad at fundamentals of RTS, like scouting early for weird builds and reacting quickly to it. Remarkably, the same rush, namely a Grizzly, is a common rush on small maps and a very successful one.

While I agree that failure to scout was the most egregious thing that went wrong in this game, a lot of other stuff went wrong too. South was in duplicate amph factories making ducks (which I do not think is a premiere unit, on this map at any rate), half-heartedly contested the supermex in the middle (I think either hard committing to contesting the supermex or the flanks is superior on this map), happened to have all the wrong units against Grizzly (though Chudy at least tried to switch into Racketeer), and moved very belatedly to either support each other or try to put pressure on elsewhere (though due to the abysmal unit matchup their entire army brought to one place probably still loses to a Grizzly and a com or two).

You are going to need to counter the big unit at some point, but depending on the unit and the map you might have some time to raid and expand first.

Icy Shell is small and has a supermex in the middle, so if their big unit controls that you are unlikely to even have an eco advantage - you do not have time to mess around. Best to counter whatever they have rushed pretty much immediately.