Here in the printer’s manuscript, Oliver Cowdery definitely wrote an e vowel for the past-tense form knew. The knew, however, was at the end of the line in 𝓟, and Oliver accidentally smeared the e, which makes the letter look somewhat like an o. This lack of clarity led the 1830 compositor to set know rather than the correct knew. In accord with the reading in 𝓟, the 1908 RLDS edition restored the original knew.
The verb know occurs in the present tense in other parts of this narrative: “do we not know” (verse 32), “behold we know” (verse 34), and “and then shall ye know” (later on in verse 36). This surrounding present-tense usage may have played a role in causing the 1830 compositor to misread the knew as know here in the first part of verse 36. The preceding instances of know use the plural pronouns we and ye and refer to those who will be sent to interrogate Seantum; for such sentences, the present tense is correct. But in this passage in verse 36, we have the words that Seantum will speak concerning Nephi’s own foreknowledge, which Seantum will refer to in the past tense (that is, Seantum will say “Nephi knew nothing concerning the matter”). Thus the use of the past-tense knew is fully appropriate in the first part of verse 36.
Summary: In accord with the reading of the printer’s manuscript, restore the past-tense knew in Helaman 9:36: “and then shall he say unto you that I Nephi knew nothing concerning the matter”.