"Generally, there is one thread for each application and one for the operating system’s user interface as well."
I think the article writer meant "process" -- there are many threads per process, and that is present in most "contemporary operating systems". If this is their (Haiku Inc.'s) attempt at PR... they are not doing it well.
There can be many threads per process, but plenty of applications ("Generally") are still single-threaded or even if they aren't, they only use one for the UI.