This is a problem of translation between a language that has articles and one that does not.
"To' nearly never means 'this' but rather 'that'. In absence of articles and if we need to point out the fact that it is a known item 'that' or 'this' is used. In reverse translations the course accepts both 'that' and 'the' as an appropriate word.
Well, "this is a new word" and "this word is new" are two completely different sentences. The first is an answer to "what is this" and the second to "what kind of word is this". So "this is a new word" would not be accepted one way or another...
But that all is slightly different issue than this vs. that. This is something that is right here, i can touch it most of the time. That is something that typically is further away or we do not see it at all. They can overlap at times but in general there is a difference between the two that could be compared to "here" and "there".
So I will think of "to" as being "that thing over there." (Except in this example.)
Re syntax, you're right, the two sentences are not the same, but I imagine they could be used to much the same effect in a given setting, which is what sprang to my mind when doing this module. Suppose you are reviewing a list of words you've just learned, when you notice a word that you don't recall having been there before at all. Might you say:
Well, this word is new. Interesting, that word is new. Hmm, that is a new word. Strange, this is a new word.
I think how exactly I would understand the speaker might just depend more on intonation than word choice or the syntax.
The system doesn't recognize it because it's not a standard contraction like "don't," for example.
I strongly recommend that you avoid using this kind of "unexpected" contraction in the future, because it likely will be rejected. Possessives should normally be accepted, along with standard contractions.
The course team just doesn't have the resources to manually add alternatives that include every possible English contraction.