Had they they actually stayed in battle they would have been roflstomped.
Listen, if healing was as powerful as you think it is, it would be a viable tactic that people use. The problem is that it clearly isn't. It is much more efficient to DPS your way through all content because DPS > healing and it's not even close.
There is no way to mitigate the massive amount of DPS in this game through healing or buffs or dodging or anything. If there was, we would see it happen and the game wouldn't constantly be stuck in easy mode.
Its a combination of stuff. healing alone will not cut it in any game if you're faced with a force 3x+ as large as you are, crowd control, support alone will also not cut with such a large force. Obviously you cannot just face it head on, stand there and expect to tank it easily.
PvE is stuck in easy mode, i completely agree there, not because healing is not viable but because of Gw2 diagonal like design. For PvE to be hard you'd need to balance stuff across the best stats hence exotic / ascended armor would then be a requirement to handly the hard content and thus gear would become a content barrier. PvE difficulty is a problem no doubt and I agree with you focusing on DPS in PvE is the most efficient way to go about it because encounters are easy enough that you dont need to bother with conditions / healing etc... fights are generally over before you loose even 1/2 your health and those are the "hard fights" Ignoring those aspects though makes you play lazy because in pvp if you just focus on DPS you'll simply be less effective than players who play in a more holistic manner. That is what happened in the video. Those players didnt simply avoid the fight, they fought intelligently and they killed a lot of enemy players.
you can mitigate 100% damage just not all the time. you can also substantially reduce the damage received by even as much as over 50% of it.