No they don't. Green cover reduces incoming damage so unless you rifle squad is already almost dead it will take 40% reduced damage and not be wiped.
Also if you miss the most obvious grenade tell in the game along with it's travel time then welp I don't really know what to say. The abusive thing about defensive stance is that it has no draw backs because you can reposition almost instantly.
It's travel time is like half a second. You have to be watching the guy crouch from the very start to effectively avoid it and in the heat of battle you cannot always be doing this not to mention sometimes when the models are just firing like normal there will be a guy crouched and firing. As well as world objects and other things effecting your ability to see the model. And you cannot always have a healthy squad I mean it takes time to get an ambulance and you don't just retreat because you lost 1 model. If I wanted to I could make you a compilation video of how many times I have had riflemen wiped in green cover. In open terrain its much easier avoided but in cover world objects make riflemen bumble around and they just cannot move fast enough most of the time.