It wasn't being stupid, no one would expect a 30 man squad to simply kongaline to block off the deployment zone completely - it's simply a very dodgy move (legal yes, but dodgy)
I would have, expected it, that is. I've been waring opponents about it ever since 5th came out, back when I was still using a 4th ed IG codex and could infiltrate whole platoons. And there's nothing 'dodgy' about it at all. Is it dodgy for me to shoot at a unit outside of cover to prevent you from getting a cover save? No difference. You make a mistake and I get to take advantage of it. It's that simple. GW recognized it back when SW Scouts first got Behind Enemy Lines, and stated that if a conga line (Picket fence was their phrase, IIRC) prevented a unit from coming onto the table the unit was destroyed. Based on that precedent, the game is not hung on turn one. It goes to turn 2 reserves, units are destroyed, turn 3 reserves, etc until the White Scars player runs out of units.
Don't forget, the Kroot player WARNED the idiot that he had a lot of infiltrators, and then even asked him "Are you sure about that" when he put everything in reserves. That's one warning more than I would have given him. All the WS player had to do is deploy ONE sacrificial model, and it could not have happened. Yeah, I'd call it stupid.
I even did it to a Kroot player in a GT practice game, back in 4th. He declared he was going to infiltrate his whole army. I deployed half of mine, we dice off, I win and get to infiltrate a unit first. By stringing it out, he now has nowhere on the table that is outside of 18" and is unable to infiltrate. Game over. We swept it clean and reset, and from then on, if his opponent had infiltrators, he deployed at least one unit normally to prevent it.