Actually, on casual reading of that code I'm not sure that it is a synchronous request. The comments could possibly be out of date.
There are 2 reasons why synchronous requests might be justified:
1. On the server side. If the request isn't idempotent, then having other requests serviced at the same time could scramble the results.
2. On the client side. If the xhs manager gets confused as to which received response it gets matches which request. Since web servers handle requests asynchronously (and often in parallel), it's possible for request A to precede request B, but get back request B's response before you got back the response for request A.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.