To determine the exact block of the snapshot, the block id of the last block produced after 0:00 UTC September 16 will be divided by the number of blocks produced from 0:00 UTC September 1st through 0:00 UTC September 16th, and the remainder (mod) of that division will represent the block number (from this date range) that the snapshot will be taken.

The wording is unclear.

Do you mean "the last block produced before 0:00 UTC Sep 16" or "The first block after 0:00 UTC Sep 16"?

What if there are several matching blocks produced (possible through forks)?

Also note that the witness signing that block has some influence on the ID and can therefore "mine" the block in such a way that a specific snapshot block is used.