Non-native disclaimer invoked, but there is a difference.
The definite article in French can be used to refer to things in general. "Liking" is a general condition. "I like thick crepes" means I like them in general. Hence, in French that idea can be expressed with a definite article (obviously, the same sentence can be used for "I like the thick crepes" if one is referring to a certain specific grouping of thick crepes.) On the other hand, "wanting" is not equivalently general. If I want thick crepes, I presumably just want a limited quantity of them.
Hence, to translate "They want thick soup" one would expect the translation "Ils/Elles veulent de la soup épaisse."
The best option is going to be to understand article usage in the language you're learning. There is nothing "silly" about "They want the thick soup." It's a different sentence than "They want thick soup," and those two sentences have equivalents in French. Duolingo sometimes seems to have an unlimited quantity of tricks up its sleeve, but I will be surprised if "They want thick soup" is ever added as an accepted translation here.
I'd think the go-to here would ordinarily be a subjunctive expression. There are a number of Google results for "veut que la soupe soit" (so here it'd be "veulent"), one of which uses "épaisse":
suivant si l'on veut que la soupe soit épaisse ou plus liquide
It's possible there is a yet more parsimonious version.
EDIT: I note that "They want the soup thick" is now accepted.
I agree. Vouloir calls for the subjunctive if the implication is that someone else is involved, as in a chef, then I should have written : ' Ils veulent que la soupe soit plus épaisse.
But if they are the cooks, is the subjunctive required? Probably as they are still speaking of others in the group.
The example I gave seems to be from a cooking site, so the "l'on" there is presumably the somebody preparing the soup. Grammatically, the subject of the subordinate clause is "la soupe," which is different from the "they" no matter who the "they" is, thus triggering the subjunctive after "vouloir," at least as far as I recall my grammar rules.
There may be some completely other turn of phrase that doesn't require subjunctive. I'd love for some native speaker to drop by and inform us of such a thing :)
What if they aren't making it but ordering in a restaurant?
That extract is a none too flattering description of how the addition of cornflour (corn starch in AE) is used in processed products under the guise of "natural additives" to "make the soup thicker". So Ils veulent rendre la soupe plus épaisse translates as "They want to make the soup thicker".
As an aside, personally, I wouldn't like cornflour in my soup and I don't think I have ever seen a recipe that calls for it. :-)
I agree it's possible one may have to give some thought to what one means by "they want the soup thick(er)" to determine a French translation. I would be quite content for a native speaker to stop by and inform us of something closer to a word-by-word version (or versions) if such exists.
If you were speaking in generalities and not about soups that were there to choose from, how would one say¨They want thick soup.¨? Would it not be the exact same when translated into French, with the context indicating whether the speaker was referring to a particular thick soup or just thick soup in general?
I totally agree with you! IMHO, the English 'the' cannot consistently be used to denote things in general. My theory is that usage in fact reverses when attached to uncountable/non-count nouns, mass nouns and such. 'Soup' means soup in general, if you want to specify concrete, actual soup in a bowl you'd say 'the soup'. Hence the translations should actually be reversed:
"They (always) want thick soup." = "They always want soup thick." = "Ils veulent la soupe épaisse."
"They want the thick soup." = "They want the soup (made) thick." = "Ils veulent de la soupe épaisse."
More a little-understood rule of English than one of French . . .
How do I know when "la" (or "le") is supposed to warrant a "the" in the English translation?? I put here "They want thick soup" because I didn't see the need for "the" but it was marked as wrong. Conversely, "She likes pale colours" was translated as "Elle aime les couleurs pales" on here so...... when do you use "the" and when don't you?
I am not a specialist! but earlier grammar units have specified that la/le/les is used not only when "the" is intended, but also when 1) the noun in question refers to a class of things in general (e.g. "Horses are great!" would be "Les chevaux sont géniaux !"), and when, and this is what is relevant in this case: 2) the noun follows "aimer" or "adorer" (presumably when it is direct object to such a verb). Hope that helps!