I doubt that it was just the translation of "entirely" that was at issue. rud eile ar fad could be "a completely different thing", "an entirely different thing", "a totally different thing", "a different thing entirely", "another thing entirely", "an altogether different thing", "something else entirely", "something entirely different", "something completely different" (rud éigin eile might be better for "something", but it doesn't really change the essence of the sentence).
When there are a lot of different ways to say essentially the same thing, it's not surprising that they haven't all been added as alternative answers.
Ithimid an rud ar fad can be understood as "we eat the whole thing", or "we eat the thing completely" but it's probably not the best way to say that. But rud eile ar fad really just means "a totally different thing" or "something completely different" - the ar fad is intensifying the eile, rather than emphasizing the totality of the rud.
In lesson 5 of Adverbs, there was i bhfad, which I thought was coming from fad because i causes eclipsis, and because we can see i bhfad appearing in the examples of sentences at the bottom of the page. So, if this is again the same fad in ar fad, why is there is no lenition (ar fhad) ? Because in the Lenition skill, ar is in the list of prepositions that lenite.
ar lenites. It also doesn't lenite, and on occasion, it even eclipses.
The entry for ar in the FGB has this to say about the matter:
(In references of a general nature it does not normally affect initial letter of following noun, e.g.ar muir, ar cíos, ar cosa in airde, "on sea", "rented", "galloping". In qualified or particularized references it lenites, e.g. ar mhuir na beatha, ar chíos mór, "on the sea of life", "at a high rent". Eclipses in a few instances, e.g. ar gcúl, backwards)
In this particular example, you will hear both ar fad and ar fhad (the current speaker for Duolingo says ar fhad in at least one exercise, though she usually says ar fad - the text always says ar fad).
In Adverbs 5 we had 'eile' and the only sentence I came across was 'Am eile' meaning 'another time'. We already know 'rud' - 'thing'. So, 'we eat thing another all' could, with a stretch of the imagination, become the answer above, but we haven't done 'completely' and it would be more natural to say 'we eat a completely different thing'