I'm not an expert on Chrome OS internals but it seems to me that being a GNU/Linux derivative with an X Server, it's not much of a jump to say the Linux version is being developed and tested first.
Edit to clarify: I'm guessing the development is being done on a Linux machine rather than directly on a Chromebook.
Chromium is actually providing it's own compositor and windowing system.
The plugin they are getting rid of the is the Chrome-only Pepper API based plugin that for some reason is unavailable on the current Samsung ARM Chromebook.
Whether or not this is actually hard for their codebase, I have no idea.
At this point it's just kind of silly to exclude Linux, especially when all the heavy lifting is already done; Chrome OS _is_ Linux, and I believe the Roku devices are Linux-based as well.
The reasons Netflix might have to exclude Linux are not unique to Netflix, but somehow Amazon and Hulu - both Flash-based, of course - manage to support these customers.