Mambo rafiki! You are completely right! So, I have already reported it. :)
These are the examples from Tips and notes:
Ugali is stiff porridge in English, but you can just call it ugali.
Pilau is a spiced rice, but you can just call it pilau.
Nyama choma is literally barbecued meat, but made in a special, smoked manner, so it can simply be referred to as nyama choma.
Mtori is literally a banana soup made with green bananas, but that is a long descriptor, so mtori is acceptable!
Report it. I guess they just didn't think of that ... fish is generally an uncountable noun in the context of food, but not necessarily, and the context is not necessarily food. Leo nilinunua samaki na ugali.* (Today I bought (a) fish and ugali ... maybe the fish is a pet.)
Yeah, BenUserName is right. There are no "a, an, the" articles in Swahili, so it probably wasn't taken into consideration when translating it into English. We are still in Beta, so feel free to suggest that as an appropriate translation, since English does have and use those articles.