Is that the case, though? My understanding was, that even if I run a docker container as root and the container is 100% compromised, there still would need to be a vulnerability in docker for it to “attack” the host, or am I missing something?
Second, even if your Docker container is configured properly, the attacker gets to call themselves root and talk to the kernel. It's a security boundary, sure, but it's not as battle-tested as the isolation of not being root, or the isolation between VMs.
Thirdly, in the stock configuration processes inside a docker container can use loads of RAM (causing random things to get swapped to disk or OOM killed), can consume lots of CPU, and can fill your disk up. If you consider denial-of-service an attack, there you are.
Fourthly, there are a bunch of settings that disable the security boundary, and a lot of guides online will tell you to use them. Doing something in Docker that needs to access hot-plugged webcams? Hmm, it's not working unless I set --privileged - oops, there goes the security boundary. Trying to attach a debugger while developing and you set CAP_SYS_PTRACE? Bypasses the security boundary. Things like that.