Big difference is Android on ChromeOS is using the Linux kernel that is part of ChromeOS. Here Android will be on top of Zircon. Which is the fuchsia kernel.

Now gnu/linux on ChromeOS is using a completely different Linux kernel.

It was important Google did it this way so gnu/Linux would still work when they update ChromeOS to Fuchsia. Google already has Gnu/linux running on fuchsia with something called Machina.