That's right -- plural suffixes in Klingon are optional. So tlhIngan can mean "a Klingon" or "the Klingon" or "Klingons" or "the Klingons", while tlhInganpu' can only mean "Klingons" or "the Klingons".
In this sentence, tlhIngan must have a plural meaning because the verbal prefix lu- shows that the subject is third person plural (and the object is third person singular).
I wouldn't call languages being lazy so much as languages being interested in different levels of specificity. In English you usually have to be very specific about a noun: are you introducing it for the first time ("a Klingon") or do you know about it already ("the Klingon"); how many are there (one "Klingon," many "Klingons"). You can't NOT say these things. Klingon lets us say things that aren't so tightly constrained in meaning: Suv tlhIngan. If I'm introducing a class to the concept of Klingons, the sentence might be introducing a generalization about Klingons: "Klingons fight." If I'm telling a story and I want to introduce a Klingon by zooming in on him on a battlefield, the sentence means "A Klingon fights." If I already know about a Klingon and he gets into a fight, it means "The Klingon fights." If I already know about a group of Klingons and they get into a fight, the sentence means "The Klingons fight." That single sentence is useful across a broader set of circumstances than in English. Likewise with the lack of tenses on verbs: in English we MUST specify a tense; in Klingon we can infer the tense from the context. If I'm telling about an ancient battle, the sentence means "The Klingon fought" (or "The Klingons fought" or "A Klingon fought," etc.). If I'm talking about something happening right in front of me, the sentence means "The Klingon fights" (etc. for number and article). If I'm predicting what a Klingon will do when he sees his enemy, the sentence means "The Klingon will fight."
The downside to this enormous flexibility is, of course, that when you really do want to narrow down what you mean, you can't do it by inflecting the words; you have to explain what you mean. If, in an isolated sentence, I want to talk about a single Klingon fighting in the past tense, I have to establish that through context: wa'Hu' Suv wa' tlhIngan "Yesterday one Klingon fought."
It's these verb prefixes - which I personally call "pronominal verb prefixes," for lack of a better term, which act the same way as verb conjugations/inflexions in many Earth languages, that allow the pronoun (and in this case, the plural marker) to be omitted. I've noticed that languages with a higher level of inflection, or conjugation, on the verb are more likely to allow the pronoun (or plural marker) to be optional, since the verb already specifies the person and number. Of course, in cases like this, where the plural marker is missing, the two would seem to contradict one another, as the number of lu- doesn't match qoq. Fortunately, we do have precedents for a singular sometimes acting to represent a plural in cetrain cases: "man," "a/the Klingon," so that works in our favor. I'm personally still getting the feel for when things such as pronouns and plural markers can be left out of a sentence in Klingon - but then, it's early in the course. :)
This report is pretty much addressed by the existing conversation on the topic, but let's make it even clearer.
Comment: "There's a typo. There was only 'Klingons' when it said tlhIngan not tlhInganpu'."
Response: It's not a typo. The word tlhIngan on its own can be singular or plural, the suffix -pu' being an option. Here the prefix lu- tells you that the subject must be plural and the object singular, so you know it is multiple Klingons who respect the one Kahless.