Yes, but that is the point...even though (assumed it was just the assasin and maybe one or two others attacking them) they could have done something, they didn't. And yes, what you meant was what made me sad too...it did not even look like they did it on purpose (sometimes this annoys a ganker more than fighting back), but actually thought they would be there for a world boss only (and obviously had not done much open pvp either)...
The "it is a pvp server" argument is false on so many accounts...no need to bring up that discussion again. Under certain conditions, true. But here obviously content was blocked...and the fault lies mostly in the design, not in those exploiting it (though if you keep doing it, it says a lot about you...killing afk, busy or chatting people must be a real challenge ). With enough decent population, there will always be people hunting him...making the whole thing again fun for two parties, not just one as shown in the video.
For example, if funcom spawns such bosses near a ress point, the "disturbance" factor of solo people interrupting and blocking content can be foreseen. Same goes for putting events into bottlenecks...
If the attacker actually was taking a risk here (like he really had to survive to get xp and kills or travel a longer way while being at risk of being hunted), but he only needed to injure and the boss and adds did the rest -> game design flaw.
Same goes for collateral damages on lowbies creating murder points for the whole raid -> exploitable game design flaw
Take white sands, it combined both bad designs ...a bottleneck and a ress...