Consider this definition for "could", from https://www.collinsdictionary.com/dictionary/english/could
Definition 16: You say 'I couldn't' to refuse an offer of more food or drink. [informal, spoken]
a) More cake?'—'Oh no, I couldn't.'
My question: What is the difference between sentence a) and sentence b)?
b) More cake?'—'Oh no, I can't.'
My comment: To me, sentence a) is conditional: "(If you gave me one more piece of the cake), I couldn't (finish it)." Do you agree with me?