> You seem to be interpreting it as an accusation based on the idea that a person making this statement about himself is very unlikely to be incorrect.
I'm interpreting it as an accusation based on the prior explanation, which is less about likelihood of correctness and more about questioning the single authoritative root source of information. In truth, that reasoning is really my explanation of what I see in practice. I cannot recall an instance where someone said "I find that hard to believe" to someone's statement about their own current actions that did not also carry a clear "I call bullshit" connotation. That is, while logically "I find that hard to believe" used in this way can mean that a person thinks you might be wrong, I find that in practice it is not used this way, so it's irrelevant in this context. Specifically, I think the statement as used hear carried a clear "I call bullshit" connotation, which is an implication of lying.
That said, I freely admit my experience in the use of this expression in English might be influenced by region, or even my own biased interpretation, and you or others may have experiences where it was used by or to you in reference to an assertive statement about your action in which there was not a clear "I call bullshit" connotation, in which case I would happily hear them and use them as counter evidence to my own experiences.
> "I don't use the default apps" could be low or high certainty.
To me, assertive statements like this do not exhibit low certainty at all, specifically because it's referencing current state. If it's about the past, it's open to recollection issues, if it's about the future, it's about possible future actions, but when you state "This is what I do", to me that is meant as a clearly defined statement of truth.
