Well, in this particular case you kill them so fast that is mostly meaningless. For that deadeye, yea, it's not very tanky. On my Slayer, with a measly ~180mil boss dps, that one could tank stuff while still doing very good boss DPS.
That PoB I cooked up is really just a "lol look at how easy it is to dump crazy amounts of DPS with this skill". Don't think I'd recommend trying to tailor a build around it. The Big Ducks video however is an actual build and is not even using a full 6 link. And Jungroan's works without a 6 link as well (I've been playing on a dual 5 link with KBlast/KBolt all season).
Most of these builds use KBlast for everything but bosses/rares so they still have good clear while mostly having decent defenses because of how fast they kill and how much leech they have.
Here is the Slayer from before I switched him over to that CWS build I was theorizing:
➤ Life: 5,071 [111%]ㆍES: 858ㆍMana: 83ㆍeHP: 45,721 ➤ Resistances: 75%/75%/75%/75%ㆍArmour: 28,825 ➤ DPS: 231,665,632ㆍSpeed: 6.21ㆍHit Chance: 100%ㆍCrit Chance: 100.00%ㆍCrit Multi: 530% ➤ Config: Pinnacle, Frenzy, Power, 15x Wither
pobb.in
^^ That is the bossing setup, in a 6 link without KBlast.
This is the mapping setup with KBlast (The flasks have enkindling orbs because I've switched to a Mageblood so the defenses aren't as good as the PoB suggests, but still good).
➤ Life: 5,071 [111%]ㆍES: 858ㆍMana: 83ㆍeHP: 51,429 ➤ Resistances: 75%/75%/75%/75%ㆍArmour: 46,880 ➤ DPS: 138,422,416ㆍSpeed: 5.91ㆍHit Chance: 100%ㆍCrit Chance: 94.67%ㆍCrit Multi: 382% ➤ Config: Pinnacle, Frenzy, Power, 15x Wither
pobb.in
I left Immortal Call unchecked, but it has almost 100% uptime on an endurance charge stacker while mapping. You can re-check it to see how much defenses it has while mapping for the most part. This build, the gear that is, is mostly quite cheap EXCEPT for the tincture, but the tincture is also build enabling. It would still be good with just a curse tincture without the proj, but it wouldn't be that level of blast your face off.
I probably should have just reposted that instead of the theoretical one, its a good comparison against Deadeye which lack those defensive layers.