Your link doesn't say that Google forces AMP on publishers. It shows that Google displays an icon next to AMP results for mobile searches to indicate the page is mobile friendly. Bing does the exact same thing: https://blogs.bing.com/Webmaster-Blog/September-2018/Introdu...
This is not enforcing AMP on publishers in the results, and the argument that it is by using icons falls under the 'it's kind of the same' category.
It says they use site speed in ranking, and, well, I'm sure it would come as no surprise if Google's (largely) having served a page/site increases the speed just enough.
This is not enforcing AMP on publishers in the results, and the argument that it is by using icons falls under the 'it's kind of the same' category.