In 2 Nephi 9:4-5, we read
4 For I know that ye have searched much, many of you, to know of things to come; wherefore I know that ye know that our flesh must waste away and die; nevertheless, in our bodies we shall see God.
5 Yea, I know that ye know that in the body he shall show himself unto those at Jerusalem, from whence we came; for it is expedient that it should be among them; for it behooveth the great Creator that he suffereth himself to become subject unto man in the flesh, and die for all men, that all men might become subject unto him.
Since the people have searched, Jacob knows that they will know the things highlighted in these verses. Presumably, by "searched" he means searched the scriptures, which for them were the brass plates. Compare with Job 19:26
And though after my skin worms destroy this body, yet in my flesh shall I see God
and the words of Zenos quoted by Nephi in 1 Nephi 19:13
And as for those who are at Jerusalem, saith the prophet, they shall be scourged by all people, because they crucify the God of Israel
In other words, since the people have searched much in the scriptures, Jacob knows that they are familiar with Job and Zenos on the brass plates. Zenos had written that the God of Israel would suffer death by the people in Jerusalem. His writings probably got lost sometime before the New Testament, so the Jews didn't know they fulfilled his prophecy, but the people in the Book of Mormon knew that this would happen.