Is it possible to do a square loop in a 20knot wind? Yes it is. You can do one that looks square with a model that'll do more than 20 knots, and also do one that doesn't look square with one can't do 20 flat out in calm. You might not be able to land back at the runway, but it's possible to do.

Is it possible for model pilots to judge the speed their model is flying closely enough to fly it at an exact 20 knots ? or any chosen airspeed? Not really.

Basically it's not possible for the vast majority of us to actually do what is described. Because the description includes things not possible to know, such as actual airspeeds as stated. And it's not possible for anyone who has actually flown a square looking loop in a wind to have known the wind speeds encountered along the path nor the airspeeds or the ground speeds.

Got a better description of the impossible feat?