Everything in the sandbox is considered user space. I worked on building one for another tech company, you start from the assumption that everything in it can be accessed by the user. The only reason the content of the sandbox is not anywhere easily accessible is because that would be poor UX and useless for 99.9% of users not because it’s supposed to be secret. So yes it’s not a vulnerability, this is equivalent to opening the dev console on a web page.
I hope they reconsider and I think you've got a good case that this was a very serious attack, second only to getting a remote shell -- and a good stepping stone to getting a remote shell if you weren't so ethical.
I think you're confusing the expected behavior of the product offerings. Every user gets their own VM for free. would you be similarly convinced an attack has happened if AWS gave you a remote shell to the instance you rented?
You can literally just ask Muse for a remote shell, it's happy to give you one! And why shouldn't it? This is about as critical as Amazon granting you SSH access to your own EC2 instance.
