I thought the scientists from the 24th century had to have been responsible for the cylinder because they were responsible for the key that opened the cylinder. They found the Progenitor tech 800 years ago and decided it needed to be more hidden than it was. That’s why they made the clues Discovery has been collecting all season. Those scientists may have followed clues left by the Progenitors themselves, but the clues Discovery has been following were left by the scientists not by the Progenitors. The clues lead to and allow the opening of the cylinder. I was thinking the portal is original to the Progenitors because it’s still operational and as we saw with the Denebulan water makers, 24th century technology can fail within hundreds of years unless it’s maintained.