This verse can only be understood in the context of the latter phrase of the previous verse. Just prior to saying that “ye may suppose that this is foolishness in me…” we have:
“…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 suggest that while there is a tradition that the plates of brass will retain their brightness, it is entirely possible that this brightness requires some human assistance, since Alma says that they are bright “if they are kept.” This suggests that some aspect of keeping might include an action to maintain the brightness.
Now we have the question of what it is that Alma supposes that Helaman will think is “foolishness.” There are two possibilities. The first is the keeping that might be required to maintain the brightness, and the second is Alma’s extension of this “bright” promise to all of the plates, not just the brass plates for which the original promise was given.
The text that follows concerns the value of the information on the plates to the people of God. It would appear that rather than see the promise in terms of the “brightness” only, Alma saw the “bright” as part of the original promise:
1 Nephi 5:18
18 That these plates of brass should go forth unto all nations, kindreds, tongues, and people who were of his seed.
This is the phrase that precedes the “bright” promise in Nephi, and Alma considers the two to be part and parcel of the same thing. The brightness is the physical sign of the symbolic value of the plates. As long as the records are kept they are preserving their brightness and therefore are fulfilling the most important part of their promise that they shall go to “all nations, kindreds, tongues, and people who were of his seed.”
Thus this “little thing” of the brightness of the plates is but a sign of the more important function of the plates. What then is the foolishness? There is really no way to know for sure. What we can tell for certain is that Alma linked the brightness of the entire plate tradition to the promise of the value of what was on those plates. For Alma, they were indelibly separated, and perhaps it is this that might have seemed “foolishness,” that the brightness or physical appearance might have any impact on the value of the contents of the plates.