AMP at the end of the day is an open source javascript library. The pages accessible on AMP via the cache, can also be accessed directly if you want. I don't see this as a walled garden - having a cache does not mean the content is not available outside the cache.

Google does not do this out of altruism of course - their margins on web search are much higher than on mobile, and if more and more content exists only in mobile apps and walled gardens they will lose revenue.