it mentions the "popup true doesn't work in iOS" in passing, however, what it doesn't seem to really mention in the accompanying docu is that if you set popup true it returns the parameters as fragments (i.e. like #access_token= instead of &access_token= which makes the rest of the tutorial moot).
Got me today anyway FWIW.
options = true;
When the user signed in on Amazon Login page and the popup window was closed, LWA SDK receives the authorize response and redirect current window to the URL you specified above with response parameters included in query string parameters. If you do not specify the callback as an URL or use the redirect flow instead, you will find the access_token included as fragment in the authorize response.