The 1830 typesetter changed the present-tense are to past-tense were, probably unintentionally. Since the biblical text can be thought of as existing in the eternal present, there is nothing wrong with the original reading, as represented by the printer’s manuscript (the original manuscript is not extant here).
The Book of Mormon text has quite a few references to the biblical text in the present tense, as either “is written” or “are written”:
In 3 Nephi, when citing scriptures, Jesus always uses the present tense, either “it is written” or “which is written” (six times in all).
The past tense (either “was written” or “were written”) is also used to refer to the scriptures:
So theoretically either the present tense or past tense is possible when referring to the scriptures. In each case we allow the earliest textual sources to determine the tense. Here in Jacob 2:23, the critical text will restore the present-tense “are written”, the reading of the printer’s manuscript.
David Calabro (personal communication) points out a systematic difference between these two lists. The three past-tense examples are each found within a past-tense historical narrative; thus the past-tense verb form was or were is appropriate. On the other hand, the present-tense examples all occur in discourse or dialogue (including two cases where the writer is directly speaking to us as Book of Mormon readers, in 1 Nephi 19:21 and 2 Nephi 4:2). Here in Jacob 2:23, we have a case of discourse where Jacob is speaking to the people of Nephi; thus the present tense is appropriate.
Summary: Restore in Jacob 2:23 the present-tense reading of the original text: “because of the things which are written concerning David and Solomon his son”.