ChromeOS is Linux, as it's running the Linux kernel. As is Android for that matter. But anyway, Google doesn't let you run random binaries you download on ChromeOS, outside of a limited sandbox, making it more secure than a distro that lets you run curl | sudo sh outside of a VM.

There are some other security shaped things they do as well.

https://support.google.com/chromebook/answer/3438631?hl=en