I think "the model escaped the sandbox" is an entirely credible description of what happened here.

If you like you could say "the coding agent harness called a model with a sequence of text which was turned into numeric tokens which were run through many layers of a neural network to produce more numeric tokens which were converted back to text which produced executable script statements which the harness then passed to a shell which resulted in commands being sent to the vulnerable proxy that chained together and caused effects on the world outside of the sandbox", but I think "escaped" is a reasonably shortened version of that.

If you don't like the term "escape the sandbox" what would you use instead?