Earlier quoted context omitted.
Virtualbox VM encryption can be useful if the host hypervisor is compromised and the unauthorized party uploads data on disk that the host can trivially access (as is frequently the case with ransomware).
I don’t understand your comment at all. Ransomware can encrypt and encrypted virtual machine. If the host is compromised at a privilege level able to read or modify the VM, the vm is also implicitly compromised.
Multi-user systems exists, compromise may be at user-level. Sure, if you have root/SYSTEM level access then all bets are off, but defense is like an ogre - it has layers.