"That is no moon!"
Translation:Tuo ei ole kuu!
This overview says that there is an exception to the negative sentence rule (see 1.4 https://uusikielemme.fi/finnish-grammar/grammatical-cases/the-partitive-case-partitiivi ) It seems that this sentence is that kind of exception?
As suspected, this is an exception. It applies to negative sentences where the verb is olla and we're talking about a countable thing. Then partitive is not used.
Vene on punainen (the boat is red)
Vene ei ole punainen (the boat is not red),(countable)
Vesi ei ole punaista (the water is not red),(uncountable)