This. The reason the dude could jump in and bum rush the boss was because he obliterated the first two rooms to the point where he didn't care about dying at all. Secondly, and he even admits this, he would've failed if that heavy ammo didn't drop when it did.
Yeah... and another thing is that I don't think that a lot of people in this game, through no fault of their own, really understand DPS windows like that. Its a shooter first after all. There ARE windows for knocking the boss down in that last room. Plenty. But having your fireteam all on the same page for it, in that kind of situation, is something that doesn't even happen in raids. Its an entirely new type of encounter for the community as a whole imo.
So basically I'm just like... yeah, its possible to DPS the boss into oblivion. Very possible even. But for guys who grew up in the FPS world, or even the Destiny one, DPS windows closing typically have very clear indicators. There are none in this fight. People can't really gauge the overall situation and threat when locked in. That's why I really suggest that, unless you're being carried by two people, you just work on what you know - which is killing mobs as fast as possible, with the right gear out, and doing damage to the boss from different angles so that >someone< will definitely see when the orbs/spawns hit the room and be able to call it out in time for the whole team to react.