The table below presents an aligned citation of verses Nu18-21:23. The alignment justifies the Rashi assertion that the underlined word they refers back to the Israelites. Rashi infers this from the highly parallel structure illustrated by the alignment. We have indicated this aligned structure by bolding the parallel words Levites, Israelites and by underlining the parallel phrases Levites serve the Temple, and "Israelites bear sin if they come near the Temple."

Summary	Verse	Text of verse (Levite)	Text of Verse (Israelite)
Levites serve, Israelites bear sin if they come near	Nu18-21:22 read across	And unto the children of Levi, behold, I have given all the tithe in Israel for an inheritance, in return for their service which they serve, even the service of the tent of meeting.	And henceforth the children of Israel shall not come near the Temple, lest they bear sin, and die.
Levites serve, Israelites bear sin if they come near	Nu18-23 read across	But the Levites alone shall do the service of the tent of meeting,	and they [The Israelites] shall bear their iniquity;

Notice how each Row in the table has the same message. The cells in the second row echo the corresponding statement in the first row. Hence the Rashi comment clarifying the terse final cell in the second row: They refers back to the Israelites. We are told that the Israelites bear sin if they come near the Temple.

By using the aligned structure we are able to participate and empathize with Rashi in construction of the Rashi comment.