No, "grunt" is not offensive; it just means that someone made a non-verbal noise.
It's not a polite noise to make compared to using words, usually, but if someone does make the noise then calling that noise a "grunt" is not itself impolite.
(But in German, grunzen is as in Hungarian: pretty explicitly comparing them to a pig, and the sound is different from the kind usually called "grunt" in English in reference to a person. grummeln, brummen, knurren would be better for the noncommittal sound, though knurren is closer to "growl".)
Yes, some sentences are contrived, and can have too many possible - correct - translations. Duo would do Better keeping things simple. It's OK to teach foreign equivalents of animal sounds learned in kindergarden, it could be useful. Most people have kids somewhere among their friends or familes.
But simple is Better. Like the "fill in the blank" type sentences, that test only 1 word or specific phrase.
Constructing a sentence such that can be translated IN BOTH DIRECTIONS, without too many possible connotations, that could change its meaning, presents an unusually knotty challange. Only the simplest, most common and straight-forward sentences can satisfy those specs.
Are you looking for ACCURATE, or LITERAL translations? If you don't specify which, then you must accept both versions as correct. Even though accurate is Far Better, and literal is mostly useless.
"Ti rofogtok" does NOT mean "do you grunt?".
Because, that is not how an English-speaking Person expresses the same sentiment. It means "is that you (guys), oinking?" (OK, I know, you want grunting, and that's what I used, even though I disagree with it).
"do you grunt" is more like "tudsz (tudtok) röfogni?"
Therefore, I think you should accept "is that you (guys), oinking?" as two alternate correct answers.