But is not what I was expecting, they should run sequentially… since they are using the same thread

What? No, what would be the point then?

Using a single thread context is useful for sharing state between coroutines without synchronization. If you need the code to run in sequence you can just ... run it in sequence.

You can easily imaging how it works if you divide program on suspension points. All the code in-between is callabacks and you schedule this callbacks to one-threaded executor. So each delay works not as a sleep, but rather, a schedule. Having one thread here doesn't play any role on how it is executed.