Translation:Are you Klingons? Yes, we are Klingons!
I can tell that the word klingon is plural because of the plural you and we, but tlhIngan itself is writen in singular.
Plural markers are optional in Klingon.
Thus, 'IH tlhInganpu' has to be plural (Klingons are handsome), but 'IH tlhIngan could be either singular (The Klingon is handsome) or plural (Klingons are handsome).
It's generally clearer to add the plural marker, at least when speaking to people whose native language usually marks plural, but it's not grammatically required.
And the course includes a number of sentences without the plural marker, when the context clearly enforces a plural (e.g. because of the verb prefix), so that learners can recognise that nouns can be plural even without a plural marker.
Correct... sort of, but that's really the wrong way to think about it. tlhIngan is already either singular or plural based on context. You're not actually "leaving it singular", you're simply being non-specific.