In Vagary and Incursion, there is a mechanic called party-hate. Once agro'd, everyone is on the hate list, as if they had performed some action against the monster themselves. This was SE's answer to the fact that a single PLD could hold half of a delve, bosses included, by himself (when Delve first hit).
What this "Super PLD" was trying to do was 'supertank', which works in most areas. If you only agro the mobs but perform no action against them, they'll hit you forever no matter how much you are healed. In many cases, even if you die, the mobs go back to an idle state, ready to agro the next person that naturally agros them.
What he should have done is agro the mobs, pop sentinel, rampart, warcry and begin healing himself while spreading out Flash and Provoke between things when they were ready to be used again.