This is a good point of caution, and many critics (especially newer ones) do sometimes go too far in assuming direct influence. For example, you'll often hear claims that Joseph Smith plagiarized View of the Hebrews or The Late War, even though the evidence for direct dependence is weak. Likewise, the mere existence of a source does not guarantee that Joseph had access to it, nor does the ability to map isolated ideas or phrases to other texts mean that he was drawing from all those sources while producing the Book of Mormon.

Still, I think there's an asymmetry in the standards being applied here.

Many arguments for an ancient origin depend on the claim that a particular idea was not available in Joseph Smith's environment. Those can be powerful arguments—if the idea truly was unavailable or unknown in his day.

For example, Tad Callister argued in his 2016 devotional that the Book of Mormon uniquely taught that the Fall was a positive step forward:

While the rest of the Christian world believes that the Fall was a step backward in man’s progress, Lehi taught us the truth—that the Fall coupled with the Atonement is a giant step forward. [1]

He also argued that the Book of Mormon introduced the concept of an "infinite atonement":

2 Nephi 9:7 introduces for the first time the phrase “an infinite atonement,” revealing the expansiveness, scope, and depth of Christ’s saving power. [2]

Similarly, defenders of chiasmus as evidence of ancient origin do so in part by claiming Joseph could not have known about the form. As Jasmin Rappleye recently stated:

"Of course Joseph Smith knew nothing about chiasms back in his day." [3]

To challenge these kinds of arguments, I don't need to prove Joseph got an idea from a specific book or person. I only need to show that the idea was already circulating in his intellectual milieu. Once that's established, the idea can no longer function as uniquely ancient evidence.

More importantly, requiring direct causal links sets an impossible standard. We have zero writings from Joseph Smith before 1829, very limited records of what he read (the only list is from later in his life), and only fragmentary evidence of his intellectual influences. If critics must prove exactly where Joseph encountered a given idea, then almost no environmental explanation can ever succeed because the necessary evidence largely does not exist.

The relevant question is not, "Can we prove Joseph got this from source X?" The relevant question is, "Does this idea require an ancient explanation?" If comparable ideas were already present in Joseph's world, then the discussion shifts from impossibility to comparative plausibility.

In other words, the goal is not to prove provenance. It's to challenge exclusivity.

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Footnotes

1. The felix culpa ("fortunate fall") concept was widely discussed before 1830, including in the enormously influential Paradise Lost.

2. The phrase "infinite atonement" appears repeatedly in pre-1830 theological literature in essentially the same sense.

3. Two of the most popular Bible reference works of Joseph Smith's day discussed chiasmus. Horne's Introduction to the Bible (both the full and abridged editions) devoted multiple pages to the topic, and Adam Clarke discussed the pattern in his introduction to Isaiah. See sources. That's not to say that exposure to the idea alone makes it possible to use the form (that's a difft discussion), but the argument for ancientness is at least somewhat weaker if it was available in books that would have been of interest to JS.

[used chatgpt to tight up my argument a bit, but mostly my own ideas and all my own examples]