In English you can say either of the following:
1a. He is fastest.
2a. He is the fastest.
Dutch also has two possibilities. The sentences above translate as:
1b. Hij is het snelst.
2b. Hij is de snelste.
In Dutch, the word "het" can mean "it", or it can be the neuter definite article meaning "the". But "het" also has a third function: It is a marker of the superlative, regardless of gender. (Compare the "am" in German constructions such as "am snelsten".)
Thus, in 1b "het" does not refer to the subject "hij" and is not "neuter"; it is simply a marker for the superlative. That is why the best translation into English of 1b is 1a -- because 1a says simply "fastest" and not "the fastest".
In the English 2a, there is a real definite article and the sentence is the equivalent of "He is the fastest one. In other words, the one is understood but omitted in 2a.
You can understand the Dutch 2b as also using a real definite article plus an adjective, with the one omitted. But now, because the article is real, it must agree in gender with its referent. And furthermore, the adjective after it must take an -e ending, as do all Dutch adjectives preceded by a definite article. So, for example:
3a. That horse is the fastest.
3b. Dat paard is het snelste.
In other words, with a neuter noun you can say either "Dat paard is het snelst", which follows the model of 1b; or you can say "Dat paard is het snelste", which follows the model of 2b.
Nevertheless, I believe my post on this page is an accurate description of the grammar of superlatives.
DL should accept "He is fastest". Keep in mind that the DL lessons are developed by enthusiastic amateurs rather than professional linguists or language teachers. On the whole they do a great job, and I am very grateful for their efforts. But some of the finer points of Dutch grammar are handled better in other sources.