(or at-least engage reverse thrust) once at-least 2 wheels are on the ground.
Not in a 172 they don't.
But seriously I think he means leaving some power on until the main wheels touch down. This will certainly lead to softer landings than chopping the power and then trying to flare at exactly the right point to achieve a soft touchdown, but, and I don't know the 172 (read the Pilots Operating Handbook) this may not be the approved technique of Cessna. The danger is achieving that soft touchdown at some considerable length down the runway due to floating in ground effect with power on. Again, read the POH and if you are still at a flying school ask one of the instructors on type who will probably be more than happy to chat it over with you over a cup of coffee (bought by you of course!)whilst the thunderstorm passes........................!
PP