They weren't using docker and the Plex software was multiple years out of data:

https://thehackernews.com/2023/03/lastpass-hack-engineers-failure-to.html

The shortcoming, which was discovered and reported to Plex by Tenable in March 2020, was addressed by Plex in version 1.19.3.2764 released on May 7, 2020. The current version of Plex Media Server is 1.31.1.6733.

"Unfortunately, the LastPass employee never upgraded their software to activate the patch," Plex said in a statement. "For reference, the version that addressed this exploit was roughly 75 versions ago."

In the LastPass case, I believe it was a native Plex install with a remote code execution vulnerability. But still, even in a Linux container environment, I would not trust them for security isolation. Ultimately, they all share the same kernel. One misconfiguration on the container or an errant privilege escalation exploit and you're in.

It wasnt containerized sadly but remember in a container you still share (albeit split by cgroups) kernel space and the kernel. Only userland is isolated.

So kernel level sploits are still a concern. Wasn't the case here but still.