It seems that in most cases in English, you would use the plural form, but in most cases in German you would use the singular form.
Since this phrase has no context, it seems that it should be plural in both languages or singular in both, but it seems that would be too much work for them. They just defined the English plural version to be equivalent to the German singular.
I can't think of a good context where I would use the plural of Studium right now.
Studie "a study, an investigation" does have a useful plural Studien.
Even if you talk about several people studying at university, I would tend to use Studium, since each student has only one course of studies (Peter, Maria und Julia haben ihr Studium abgebrochen "Peter, Mary, and Julie quit their studies / dropped out of university") and German often uses the singular in such cases, in a distributive sort of way.
And for several courses of study at a university, I would use Studiengänge, the plural of Studiengang.
I think the problem is that Studium is a pretty abstract noun, so it's hard to conceive of a plural -- it's not really something countable.
Perhaps you might use it in a context such as Seine Studien haben ihn bis nach Indien geführt "His studies carried him as far as India", where it's not an organised course of study at a university (which is more what ein Studium is) but more general self-directed study efforts, which could be plural since they are not uniform/homogeneous.