In modern English we expect the full adverbial form surely; thus the editors for the 1920 LDS edition replaced the original bare adverb sure with surely. It could well be that sure actually is an error in 𝓟 (the earliest extant source for this passage). There is a similar example in 3 Nephi 23:2, and there the text reads surely: “for surely he spake as touching all things concerning my people”. 𝓞 is not extant in 3 Nephi 23, but both 𝓟 and the 1830 edition read surely (3 Nephi 23 is in that portion of the text from Helaman 13 through the end of Mormon where both 𝓟 and the 1830 edition are firsthand copies of 𝓞). Yet there are examples in the earliest text of the bare adverb, such as plain rather than plainly (see the discussion under 2 Nephi 25:20). For a general discussion of the bare adverb, see under adverbs in volume 3. The critical text will follow the earliest extant sources in determining whether any given adverb is bare or ends in the expected -ly.
Summary: Restore in Helaman 12:15 the original bare adverb sure, the reading of the earliest extant source (here the printer’s manuscript).