Interesting...so I watched just the first couple of examples but will watch the whole thing again as well as the whole match later.
The first instances are simply not comparable. Sexton would be the first to admit that he flopped big style in quite a comic way. Without having watched whole thing I can't comment definitively on whether sealing off took place, BUT: what the video author isn't appreciating is that the clear-out very frequently will RESULT in players off their feet as part of the contest, therefore the referee makes a judgement about whether there was attempted clearout by players on their feet, or whether they ended up off their feet as part of a contest. Thats why referees tend to say "straight off your feet" to explain a penalty because the distinction is that a legitimate attempt at clear out and otherwise.
In the lydiate case he lost his footing trying to clear out O'Brian rather than going straight off his feet. As long as he doesn't continue on the ground that's fine.
Also, when myself and Olyy complained against some early decisions against England at the aviva, it was labelled 'pathetic' - just saying...!