My problem with using "whatsontap" is that it implies everybody saying it is stupid.
The Japanese staff can communicate with the people of the world without needing to learn their language; that means there's some sort of translation ability in play for spoken language, where the people of the other world can speak their language and the Japanese Nobu staff will understand them, and vice versa.
Using "whatsontap" implies that nobody saying "whatsontap" is able to realize "whatsontap" is just "what's on tap" said together. It makes everybody seem dumber for doing it.
"Toriaezu nama" untranslated, on the other hand, can be used as a proper noun the way the patrons of the izekaya do without them coming across as stupid; of course they don't know what it means, it's not even in their language, so of course they think it's a brand because it's a proper noun for them.
You'd be surprised, that's how some places and words got their origins, people mishearing others and keeping it.
Yes they would understand, but the natives said that the Japanese staff have an accent. Meaning to them, the Japanese are speaking their language. There are also Japanese words that seemingly didn't get translated to the other world's language, examples like 大将(Taishou) became katakana タイショー, おでん(Oden) became オデン, and such. Practically the whole menu was written using katakana when spoken by the natives. As such, one could assume that whenever a Japanese word was written in Katakana in the raws, it stayed Japanese for the Aitheria natives.
Like I said, I wasn't really defending it. It was only due to the context that when another Japanese person used it as a phrase, using "whatsontap" worked. To the Aitheria natives, the Japanese were speaking their language with an accent, and yet the phrase "toriaezu nama" remained Japanese. We could then assume that to the Japanese staff, the natives would be speaking in Japanese. To them, the natives are saying "what's on tap" with an accent.
The whole point in this case was that the usage of "whatsontap" was more for the readers. I mean yeah, they could've just slapped a TL note on the same page but they went against using "toriaezu nama" because apparently the official translations used "whatsontap", so that's that.