Those are grammatically two completely different sentences.
In this fridge is cold the subject is this fridge and you describe it as cold (cold is the predicate). That’s
Tha am frids seo fuar
in Gaelic (tha is the verb is, am frids seo is the subject this fridge, fuar is the predicate cold).
In the other one, this is a cold fridge, the subject is just this and you define it as a cold fridge, that’s
Seo frids fuar
in Gaelic (seo is the subject and also incorporates the verb – it means this is, frids fuar is the predicate a cold fridge).
fridge and refrigerator are synonyms in English, they denote the same exact thing – so Gaelic frids is ‘a word for “refrigerator”’. If it wasn’t accepted, then the contributors might just not have anticipated it when creating the course. It’s hard to predict all valid translations users might try. Perhaps worth reporting a missing valid translation then.
Well it seemed fairly clear to me. However, if you didn't hear the f then it could equally have been frids fhuar. Since it is a recently imported word - indeed many people probably think that are using the English word fridge, the gender will be pretty arbitrary in practice, whatever the dictionary says.