That's incorrect. All current and recent-ish Chromebooks get at least eight years from date of release. And with the move to separate Chrome (the browser) from the OS, it will be possible to update the browser even after system upgrades are finished.

Google uses the term AUE (automatic update expiration) for the end of service. They provide a list of AUE dates here:

https://support.google.com/chrome/a/answer/6220366?hl=en#zip...