The safety cell was not compromised.
The problem here is the armco barrier. It should not split apart, but it did. If the armco does it's job, the car's nose gets crumpled and bounces off, the kind of crash you see every few weeks.
Because of the fault with the armco, Grosjean's car went through the barrier, the halo protected his head, which saved his life, and then when air intake hit the barrier the car split in two - Engine and rear on one side of the barrier, Grosjean in the safety cell on the other side.
This is a 2014 safety cell diagram - pretty much identical to today's minus the halo.
View attachment 6018
Here's the part that broke off - basically everything not in the survival cell.
View attachment 6019
That's exactly what the car is designed to do.