Which target size do they have in the clip? They're technically in cover but not from the direction of fire.
Their target size becomes 0.8 when in cover regardless of direction. They were stationary too so there was nothing in that clip they couldn't do before. The Gren models happened to be focus fired one by one (first one because it was the first one coming around the corner and then all 4 IS models happened to focus down the other models one by one, which is rare but it can happen) which is some lucky RNG/circumstances that any squad could get. I don't know how one clip is supposed to prove anything. It's like when people took that Tightrope KV-1 vs Panther clip (which was an extraordinarily bad RNG streak for the Panther) and took it for fact that a hull down KV-1 could reliably beat a Panther.
The bare minimum sample size for anything even remotely conclusive is running 20-30 tests, ideally closer to 100-200 but obviously no one is going to take the time for that. Basing arguments off 1 single video is senseless.