On regards to the video:

It should be noted that Isis gave 4/5 members a Vigor buff which is an insane buff when it comes to bursts. It doesnt work like an Atk buff as it has a far higher impact on your damage increase. Basically, if you have a 30% Vigor buff, you are literally doing 30% more effective damage. Which is obviously lowering the needed power to reach the burst cap. It also effects the Burst damage buff, so it's effect is insane.
That's why Ea for example didnt need it, since she is using a Stun Punish she reaches cap already, but it helps anyone else that doesnt instantly reaches it.
Also, Atum is working slightly differently then right now for Taco. She does increase the Ability Damage cap aswell as the general attack damage cap.
Usually, Sieg wont reach 1.1m damage with her nuke. Her Burst is also over the usual Burst cap, which lets her reach 2.8m (I think her cap at that point would be around 2.2 - 2.3m).
This is a pretty precise setup and if something doesnt go right, it instantly falls apart (that's why Isis and Atum have to die).
Another note is that Echo has a softcap aswell, so it can go beyond it. Same rules as usual that every damage beyond is reduced by 90%.