> Now it's three months since the post and nobody has pointed out the error, despite Reddit math being a self-selected crowd with a specific interest in the topic.

/u/GemOfEvan pointed out the error in the second highest (sorted by "Best") reply:

  I think I'm missing something. Alice has a message m and a
  product of primes a. She sends Bob the product ma. Bob has
  the product of primes b and sends back the product mab.
  Alice divides by a and sends back mb. Eve has heard the
  products ma, mab, and mb. (ma)(mb)/(mab) = m, so Eve now
  has the message.

Glancing through the other replies, I see at least a half dozen others also pointing out the problem, and also several explaining what it takes to make this kind of three way exchange actually work.

Reddit only reports timestamps to the month on articles this old, but all the replies I mention above say "3 months ago".