Translation:Are you Klingons? Yes, we are Klingons!
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.