I was in the Sistine Chapel on 5 May and observed tour groups exiting through the shortcut exit on the far right corner of the chapel.
The group leaders were counting their members carefully through under the direct control of the museum guards who bodily blocked the door before counting the next group through.
So, I doubt that any non-tour visitors could sneak through.
Quite a time saver for group members though.
I was in the Sistine Chapel on May 5 as well! I wasn't as observant so didn't see tour group members exiting that way but nice if that is occurring again.
At the same time, Walks of Italy, The Roman Guy, and Through Eternity have not changed their text stating that the passageway is closed, and I can't find a tour anywhere else which touts entry to the church this way. The Vatican itself doesn't have one that I can find.
So, until tour companies revise that text or are selling a specific museums + basilica tour which clearly states that this is the route they'll be following, I'd contact them before booking if this benefit is really important to you.
Example from Through Eternity's "Private Experience: Sistine Chapel, Vatican Museums & St. Peter's Basilica":
Important: since the passageway between the Sistine Chapel and St. Peter's Basilica is still closed due to COVID, we will enter the Basilica via St. Peter's Square after our visit to the museums so the Basilica remains part of the itinerary. This means we must pass security again and the tour length could be extended by 45 minutes on average.
