True Statement, yet still an Argument from Incredulity?

This is just an example I made up:

Context - Police searched Bob's home and find a stash of illegal firearms in Bob's possession. Court proceedings are later held on how Bob got his hands on such weapons. For the sake of this example Bob doesn't confirm or deny anything.

Witness with insider information: The Italian Mafia helped arm Bob by selling him weapons for dirt-cheap.

Bob's Defence Lawyer: That's nonsense, your claim is unfounded! Why would Bob spend his hard-earned money? If he wanted such weapons he simply would've gotten them from his brother Steve who is a major gun nut and lended his high-powered weapons to other members of his family, including Bob.

In this case, the Defence Lawer's claim regarding Steve lending his weapons out to others is true. Bob's Defence Lawyer doesn't believe the insider witness' testimony because Bob could've had other avenues for obtaining his weapons. So is Bob's Defence Lawyer committing an argument from incredulity here, or does he have a good enough reason to completely disregard the insider witness' story?

With just the information you presented, the defense lawyer's claim "your claim is unfounded" is accurate. The insider simply made a claim and has not provided support for the claim, so the defense lawyer has every right to be incredulous.

Dr. Bo is exactly right about this.

As an aside, I am always suspicious of the argument from incredulity. The argument assumes that the defendant isn't stupid. Hate to delve into politics, but in the 1972 election, people asked, "why would Nixon cheat when the polls showed that he was already winning by a landslide?" The question is valid: Nixon was winning by a landslide and didn't need to cheat. But he cheated anyway and tape-recorded himself doing so. For a US president who had a successful career as a lawyer to tape-record himself committing a felony was very, very dumb. But Nixon did it. American's dumbest criminals, of which Nixon was one, are, well, dumb.

We hear arguments like, "why would Joe commit adultery when he has such a charming wife?" But, sadly, Joe foolishly did commit adultery. Why would giant company X cheat its customers when they have such a huge profit? But, what a shame, sometimes giant companies cheat just for the fun of it. Why would Boeing take shortcuts with the 737MAX when their sales were so good and their reputation means everything? But you've read the news about the MAX, haven't you?

Dr. Bo is spot-on right about your example, but the argument from incredulity is often unconvincing.

I want to thank you for drawing attention to the Argument from Incredulity, OR, should this be "the Appeal to Incredulity." I had not thought of this argument or appeal before - I see it in world and national events where the application of it is used to nullify certain explanations for events that have occurred of "incredible" nature.

Your hypothetical example is okay, but here is a clear cut real event example that demonstrates the Appeal to Incredulity very clearly.

(1) WTC building 7 was not hit by a plane yet collapsed in its own footprint at the speed of gravity with visible ejections of pulverized debris coming out along the vertical support column areas. -- Controlled demolition is the only explanation for this.

(2) It is incredulous that those in some realm of authority would do such a thing. (how, why, for what reason would someone do this - I just cannot believe it) -- [yet it happened.]

(3) Because this is incredulous, "they" (whoever is being accused) therefore did not do it. (even though if one views the collapse there is no other explanation)
https://www.youtube.com/watch?v=Mamvq7LWqRU
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Study finds fire did not cause the collapse:
“The principal conclusion of our study is that fire did not cause the collapse of WTC 7 on 9/11, contrary to the conclusions of NIST and private engineering firms that studied the collapse. The secondary conclusion of our study is that the collapse of WTC 7 was a global failure involving the near-simultaneous failure of every column in the building.”
https://www.foreignpolicyjournal.com/2019/09/06/the-official-story-of-the-collapse-of-wtc-building-7-lies-in-ruins/
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Yeah, John,
Another example along similar lines.
Since there are no whistleblowers the official story must be correct.
It is incredulous to think otherwise.

In legal proceedings, for the defense, there doesn't need to be certainty, only reasonable doubt. If the statement about the Italian Mafia is unfounded, saying it is unbelievable is not even necessary. All the defense needs to do is present a reasonable alternative explanation of the facts.

I know you said the situation was made up, and it is a bit contrived. If the firearms are illegal, it doesn't matter where he acquired them. If the whistleblower doesn't have evidence to support his claim, it also doesn't matter whether or not the defense believes the whistleblower's explanation is incredulous.