The special property that encryption functions have compared to hashing functions isn't that it is extremely difficult to discover the source, but rather almost the reverse -- that for every encryption function there exists a function (decryption function) by which you can recover the unique source.

Hashing functions in general do not have an inverse function: while you might be able to recover several possible sources from them (and this might be easy or difficult), you cannot recover the single source, because the space of inputs is larger than the space of outputs, so there can be no unique mapping from outputs back to inputs that would generate them.

There's a fundamental difference between storing a password so that you can read it again (encrypt implies this), and storing it so that you can only verify it, not read it (hash). But a broader criticism of the article is that it is far too sweeping in its judgements based on scant knowledge of the topic - the little mistakes are just indications of that.

It's fine to be a beginner asking questions and the mistakes are not really so important, but it's not really useful to attempt a definitive summary of a field which you know very little about.

You can use OAuth for authentication, but that is specific purpose for Authorization.

Google has a separate product for Sign In:

https://developers.google.com/identity/sign-in/web/