Some of those shown in that stream were from Playstation, not devs. While there were some devs on, they stated during the stream that there's devs that play the harder difficulties (7+) and some that solo level 9.

Thing about the charger is right, if you use something like a EAT and are able to headshot it. The Breaker incinerary being nerfed did not gut the gun, but made it more skill intensive in terms of ammo conservation rather than point and mag dump in a given direction.