Cool. Very illustrative. Adding an it to the English sentence would actually weaken the translation slightly, because it would add specificity that is not in the original sentence (presumably). Ditto with adding that. What is known is vague and entirely unspecified (not even indefinite). The het is therefore grammatical, rather than semantical (if I have this right).
Or, if the het is also semantical, then it seems necessary to include it in the translation to communicate indefiniteness and distinguish from that.
Apology for the precision, but as a professional translator, these nuances are often the "things that matter". Of course, I will understand these nuances in due time, from an advanced course, but Duo has been a good way to form a base.