He says not to blocking-wait for async methods, which is correct.

He makes no direct comment, afaik, on calling sync methods from async methods. The reader is left only with

  efforts to be async amount to nothing unless the entire call stack is async

and

  it is best to go all in, and make everything async at once.

and must somehow parse this as some of the call stack, and make some things async.

If I made mistakes feel free to file an issue or even send me a PR. It's open source! That said, you're right that I don't say how best to call sync from async methods (the latter is more difficult as there's no good way do it well).