Alma is stating the supposed “foolishness” only to deny it. The referent to foolishness is his statement: “if they are kept they must retain their brightness; yea, and they will retain their brightness; yea, and also shall all the plates which do contain that which is holy writ.”
The conjunction of these two passages suggests that, while there is a tradition that the brass plates will retain their brightness, the conditional “if” suggests that this state will require some human assistance. What might Helaman think is “foolishness”? First, the “keeping” required to maintain brightness, and second, Alma’s extension of this promise to all of the plates.
The text confirms the value of the information on the plates for Yahweh’s people. Alma saw the promise as including, not only “brightness,” but also mission: “These plates of brass should go forth unto all nations, kindreds, tongues, and people who were of his seed” (1 Ne. 5:18). Brightness is a physical sign of the plates’ symbolic value to “all nations, kindreds, tongues, and people who were of his seed.”
Thus, this “little thing” of the plates’ brightness is only a sign of their more important function. For Alma, the traditional brightness was inseparably connected to their value. Perhaps he meant that it might have seemed “foolishness” that their brightness or physical appearance was related in any way to the value of their contents. If so, he is confessing to having a poetic soul, one that resonates to symbolism.