Well, on one hand you've got good players that have been playing for example elementalist since beta, and are still learning new stuff. And then we got you who says "I played it a short while, got it all figured out. That in the videos there can't be done".
There is an old chinese proverb (according to the internet) that says something like "Those that say it can't be done should get out of the way of those doing it" .
You're correct that 100 blades is the highest damage scaling single skill in the game, in a direct stand-alone comparison. But that's not what's being compared. Besides, 100 blades is not a good skill. It got ramp-up time, it roots you, and it spend a lot of time delivering those hits. And if you miss the last, well... Most of your "burst" just went down the toilet. And most of the other greatsword skills are pretty low on damage.
What the elementalist in the first video did was comboing several things together resulting in very high damage very fast. One of the attacks he showed there was first using Burning Speed to engage, set the target on fire (and hopefully give a crit, which have a 60% chance of causing vulnerability), then start throwing Fire Grab, switch to water attunement while casting, trigger the weapon swap sigil - thus guaranteeing a crit for Flame Grab - and get extra damage bonuses. If target is vulnerable, and he got his boons up (which he usually have) that can be over 50%. Plus the Fire Grab bonus for target being on fire. With 110% extra critical damage. And while that's a potent combo, it is just one he got in his arsenal. He mentions that just by switching to air attunement he drops a lightning bolt on target that can crit for 7k damage. And that always crits, because of the sigil.
And the lightning hammer video is as un-gimmicky and simple as you can get. It's based around the high damaged auto attack on the hammer. Then it's just to build around max'ing it (like using the water attunement dmg boost on vulnerable foes) and start swinging for massive damage.
And the third WvW video shows that elementalists are quite capable in pvp. In fact, D/D elementalists are considered borderline OP by many in PvP.