When Moroni seals up the records, he is not referring to the sealed portion of the plates, but that he is about to seal up the record in a stone box. The box…was formed by laying stones together in some kind of cement. In the bottom of the box were laid two stones crossways of the box, and on these stones lay the plates. The final seal placed upon this glorious work was a large stone lid. This stone was thick and rounding in the middle on the upper side, and thinner towards the edges, so that the middle part of it was visible above the ground, but the edge all around was covered with earth (JS—Hist 1:51-52).
But Moroni was also sealing the record in the priesthood sense. What had been sealed on earth by priesthood power could not be unsealed except by the same power. This is the same priesthood seal that Nephi placed on his last words, declaring, what I seal on earth, shall be brought against you at the judgment bar; for thus hath the Lord commanded me, and I must obey (2 Nephi 33:15).