[Half-Sync/Half-Async]: Why is the UI-Thread a asynchronous Thread?

I have to hold a lecture next month and I have some understanding problems with the Half-Sync/Half-Async Pattern.

From following source: http://ift.tt/2gXBf5z (please remove the underscore in h_ttp, i can not post more then two links at the moment)

I understand that Half-Sync/Half-Async pattern decouples synchronous Threads from asynchronous Threads.

And on this source: http://ift.tt/1OhLive

I think it is well described what synchronous and asynchronous is.

Now the core Problem:

In the lecture from Douglas C. Schmidt (https://youtu.be/S5y_jIThY9Y?t=28m35s) he put the Android UI Thread in the asynchronous layer. But I think that this is synchronous, because the UI Thread is processing my tasks sequential (the code I wrote in the MainActivity).Or I want to ask in a different way: Why is the MainActivity in the asynchronous layer?

Certainly some knowledge is missing me about this topic to understand why this is so, but I don't know where to look further.

Please can someone help me with a explanation or can give me a hint?