"As such, historic is used to describe people, things, and events that are or will be considered important by future historians, while historical is used for people, things, and events in the past, whether important or not. A "historic event" is an important moment past, present, or in the future; a "historical event" is some moment in the past."
According to the above then, "This is a historical moment" means "this is a moment in the past", which doesn't make much sense (unless we have time-travelled). Also, "This is an historic moment" means "this is an event that will be considered important by future historians", which makes much more sense.
Did you also pick up the use of "an" as the indefinte article before "h". It used to be the only correct version. Although it is not often heard now in conversation, it should still be allowed as an alternative as it will frequently be seen in literature.
That does not apply only to this sentence, of course. Applying it across the whole of Duolingo might be a challenge!
 On reflection, I have never seen "an hat" or "an house", even in literature; so it may be more of a challenge than I thought! Maybe you shouldn't bother!
You wouldn't use 'an' before all words beginning with 'h'. With 'hat', because it has a hard 'h' sound (like a consonant) at the start, it's 'a hat'. The same goes for 'house'. 'Hotel' can be said with a softer 'h' so could be 'an hotel' (with the 'h' sounding more like a vowel).
Yes, I think I added that one a while ago. Apart from the comments in this thread, there were also a bunch of reports with "an". Since we're not teaching English, I don't think it's a problem to accept both here. There haven't been many complaints in other sentences, as far as I know.
When I saw this I knew the comments would be about "a" vs "an."
General rule as I understand it: us "a" if the following sound is a consonant (such as hat or h-istoric), use "an" if the following sound is a vowel (such as 'at or 'istoric). If you pronounce the "h," you drop the "n."
Here's data on real-world usage of "a" vs "an:" https://books.google.com/ngrams/graph?content=an+historic%2Ca+historic&year_start=1800&year_end=2008&corpus=18&smoothing=5&share=&direct_url=t1%3B%2Can%20historic%3B%2Cc0%3B.t1%3B%2Ca%20historic%3B%2Cc0
What it often boils down to is whether the first syllable is stressed or not. The first syllable is unstressed in "historic," which results in the h not being pronounced much like an h. In "history," the first syllable is stressed, so the h is pronounced. You wouldn't say "an history."
Of course, as always, it also depends on what dialect you speak.
It also boils down to whether it's a front vowel or back vowel: "an historic moment", "an hysterical woman", "an hyperchondriac", "an hindrance", "an hidden agenda", but "a hat", "a hut", "a hound", "a hundred."
This is the same distinction that affected Old English G into becoming Y (geard > yard, eage > eye, giellan > yell (contrast with related back vowel 'gale')), it also affected Old English C into becoming Ch (cyric > church, ceap > cheap/chap, cylle > chill (contrast with related back vowel col > cool)).
Now with French words this same rule affects our pronunciation of G as Dz and C as S. The front/back vowel distinction is one of the most important things in sound development.
What should the sound of "sch" be here, exactly?
I know that at the beginning of words it should be spelled /sx/ (x being IPA for the same phoneme as the "ch" in toch, doch, etc.), but here I expected it to have an English "sh" - ʃ - sound, but I can only hear a faint s (as if it were a word ending in -is).
Does anyone have any clues regarding this issue?
I imagine it may vary depending on which dialect of Dutch you're speaking, but in my experience it's just an [s] sound. I know when I started taking Dutch classes in Belgium, the little bit of German I'd studied years before kept interfering and I'd say [ʃ], but it's not that. So that's Flemish, anyway, but I lived with a Dutch family, and I'm fairly certain they pronounced it [s] as well.