They do say "Flutter is a great way to build Chrome OS apps" and they say "Because Chrome OS runs Android apps, targeting Android is the way to build Chrome OS apps."

The article is about chromeos and flutter, to make it clearer, they could have said, "targeting Android is the way to build, Flutter, Chrome OS apps."

Targeting Android to create Flutter chromeos apps currently seems to be the go, this may change once flutter for the web and or the linux desktop support is fully baked.