"There are exactly one thousand people on the property."
Translation:Der er præcis et tusind mennesker på ejendommen.
Totally guessing here but I suspect it might be that folk is a collective noun in a sense, its meaning closer to 'a bunch of people' or people in the sens of "Italians are a quiet people" for instance. In which case it would be uncountable, similar to gents in French, gente in Italian, folk in english, etc.
This makes perfect sense to me! Of course, you could say "folks" in English ("there are a thousand folks here"), but the examples from latin languages make a lot of sense. It is also gente in Spanish as well. One would never say "hay tres gente" you would say "hay tres personas".
Could also be that mennesker is used for large numbers and personer for smaller numbers of countables