No, you actually don't.
You gain knowledge that some of the code is maybe publicly available, just like linux.
The rest, however, is often still kept secret, legally or not, and has been since time immemorial.
You don't have to take my word for it though, go ask bradley kuhn if he thinks that vendors have a tendency to comply with the GPL for the kernel :)
Consider what happens if you're wrong. What happens if the GPL actually is the reason why Linux has such wide open-source hardware support? Then say goodbye to hacking on Fuschia on your own hardware...
You're really not convincing me that Google's intentions are good. Not licensing under GPL is a clear signal that Google is prioritizing hardware partners' needs over their users. Just like Android.