What do you mean? It's an arbitrary RCE - a scenario that's generally treated as game over. What specifically are the concerns you don't have if that happens to a system of yours?
It's very tricky to create and maintain those kinds of container systems, but there are services (for instance, the cloud CI providers) that do it.
On applications like these, where the premise is that you give them some kind of code and they evaluate it for you, escaping the "user interface" sandbox isn't game over; you'd have to finish the exercise of escalating to the container host or getting access to an internal network with internal APIs on it.
It's a little like getting SQL injection, but confined to some kind of SQL view. Chances are the application is doomed, but you still have to prove it.
Now, I'm not saying that containers are super tight by default. It is entirely possible this particular container env is wide open, but I didn't really see anything too concerning from the parents analysis.
It means pretty much everything. Have you ever heard of someone reporting an RCE in a major service and it being treated as no big deal? They're invariably treated as catastrophic compromise because it is. The jump from RCE to privilege escalation, escape, etc is nothing compared to the actual RCE-ing.