meaning of "unbelievable assertion"

Question

In one of my posts ("that's a very good one to know" vs. "that's good to know") a nice answerer gives 2 examples and corresponding explanation

Speaker A tells a jokes, speaker B likes the joke, so he says ,"that's a good one" in response.

"a good one" means "a very funny joke". I understand this part clearly.

Speaker A announces that he's managed to fix the problem with the software, Speaker B acknowledges it by saying, "that's good"

"that's good" means "an unbelievable assertion". I don't think I understand this part correctly, so I looked it up on a dict and got

a statement that you strongly believe is true

I certainly don't agree with his assertion that men are better drivers than women.

with this definition, "an unbelievable assertion" seems to mean B dose not believe A, or at least dose not believe A strongly.

Which does not make sense. If B dose not believe A, why does B have A solve the problem and acknowledge that?

Could someone please give a hint? Thanks in advance.

Answer 1

The answer that you referenced is in error as I noted in a comment there.

That's a good one

is used ironically in certain social contexts to indicate disbelief. In general, however, its meaning is straight forward and literal and indicates approval. Although it is frequently used as a stock phrase in certain social contexts, its use is not limited to those contexts.

So your first source of confusion is a poor answer to your previous question.

Your second source of confusion is the dictionary that you used for a definition of "assertion." I wish you had cited the dictionary word for word because your interpretation of it is misleading. Here is the definition from Webster.

1. insistent and positive affirming [of something]

2. a declaration that something is true

Notice that the two meanings are distinct but related. One is a simple statement that something is definitely true. The other supplements that meaning with the additional information that the declaration is made forcefully. In neither case does it mean that the thing asserted is true or believable. In fact, it does not mean that even the person making the assertion believes it to be true.

If I say that I am the King of Siam, that is an assertion that something is true about my relation to Thailand. You may not find that assertion believable.