Earlier (in 2 Nephi 1:4) Lehi revealed that Jerusalem had been destroyed; thus the use of the past tense in 2 Nephi 6:8 is wholly appropriate. Oliver Cowdery initially wrote are in the original manuscript here, but this error may be the result of Oliver (or Joseph Smith, who was dictating the text) having gotten used to the present-tense phraseology in “they which are at Jerusalem”:
In 2 Nephi 6:8, the supralinear correction in 𝓞 to were is in Oliver Cowdery’s hand and was written without any change in the level of ink flow; thus this correction appears to be immediate.
Summary: Maintain the use of the past-tense were in 2 Nephi 6:8; the past tense is appropriate at this point in the narrative since Jerusalem has now been destroyed.