This thread, and this whole debate / subject is one that I find absolutely fascinating.
There is no universal agreed-upon definition of the different classes of tea. For example, recently on my blog I raised some questions
about how to classify white teas. Black tea and green tea tend to be a bit more clear-cut, but as your question about Darjeeling suggests, it's complex.
Classifying something as oolong is actually troublesome. It is true that oolong is a semi-oxidized tea, at least usually (some green oolongs are barely oxidized at all though, and other oolongs are so oxidized that they are effectively fully oxidized). But there is no clear consensus that all semi-oxidized tea is oolong. Oolong tends to be used more specifically, to refer to a certain type of tea, with a fairly specific, involved production process, that (usually) is semi-oxidized. Semi-oxidized teas can be produced by many different methods that few people have ever suggested are oolong. I like to describe these teas as "oolong-like". This is the text and phrasing that I use on RateTea.net and other sites where I write about these teas.
For example, many white teas (like the examples I give in my blog post) like Shou Mei and even Bai Mu Dan are semi-oxidized, and in fact, all white teas are at least faintly oxidized because the lack of higher heating allows the oxidizing enzymes to persist longer than in green tea (hence why white teas do not have the greener color). And of course, many Darjeelings and other Himalayan teas typically classified as "black tea" are also semi-oxidized. Then there are Pu-erhs, and other hard-to-classify teas like Moonlight White, which is also semi-oxidized. None of these teas are classified as oolong, to my knowledge, by anyone.
I would thus describe lighter Darjeelings and other lighter Himalayan teas, moonlight white, and shou mei as oolong-like, and as semi-oxidized, but I would not classify them as oolongs. But I'm open to these classifications changing over time, both as I learn more, and as the landscape of tea culture shifts and changes. As semi-oxidized teas become more prevalent, it may make sense to coin a new term too or a new category, or to some day broaden the definition of oolong.