Both skiers are flexing their legs, hence both are using flexion. Both skiers are resisting forces with their legs so both are also using extention. Because extension and flexion have overlapping qualities across the same plane we cannot determine what is actually happening regarding the intent of the skier.
R.M. on the other hand, is not really releasing his CoM nearly as much....
well, we surely don't know their "intent", but we can see clearly what they are actually doing in the video. The video doesn't lie. Again, look closer. Look at specifically which leg is being flexed or extended and at which part of the turn phase, etc. KM flexes his inside leg at a different time and for a different reason then Reilly.
Regarding the release, which means...releasing the CoM out of the inside of the old turn so that it can crossover the skis into the inside of the next turn:
KM is not flexing to release at all.
Reilly is.
Its as simple as that.
Its also worth noting that KM seems to primarily be skiing a longer radius turn then Reilly in these two videos.
Regarding the long leg short leg... what is that exactly? its a separate element of skiing and occurs at a different phase of the turn then releasing does and different technical models will approach it differently, but in dynamic skiing we're going to see the inside leg short and the outside leg long through the belly of the turn. Both skiers do accomplish this, but in different ways.
KM finds himself at the end of his release with both legs long. He has only one option for progressively increasing edge angles in the new turn, which is to lower his CoM into the inside of the turn while collapsing the inside leg aggressively. This is what you see as top-down, I believe, but as I said before, it has nothing at all whatsoever to do with the "release" and thus can't really be compared to what Reilly is doing during release. KM has no flexion at all during release. Reilly does. both skiers are releasing out of the old turn effectively but in totally different ways.
After the release, Reilly finds himself with both legs flexed, he has to extend the outside leg to develop long leg short leg. Its possible he may also continue to flex the inside leg even more during that phase in the same manner that KM is doing, top-down as you say, it depends entirely on how deeply he flexed during the release.
Back on topic...you have to ask yourself what is the technical model that leads one skier to perform one way and the other skier to perform the other way? The primary difference I see there is that one skier (Reilly) flexes to release and KM does not. KM uses an extension move to release his CoM and cross over, he does not use flexion at all for that. Both skiers are flexing their inside leg to develop long leg short leg and increase edge angles throughout the new turn.
Clearly there are two different technical models being employed there.
I find it relevant also that the up and over skier is skiing a longer radius turn and the flex-to-release skier is skiing a shorter radius turn. I feel that the technical model being employed by KM is more conducive for longer radius turns and I suspect he would not look nearly as smooth and fluid using the same technical model on SRT turns because not enough time to do it. Reilly's technical model can translate to longer radius turns and it would be interesting to see his medium radius turns compared directly to KM's.