This almost confirms it then, each machine has an external modem tied to 1 phone line per modem, and there are no internal modems in use. The picture shows 50 modems that I can see, and the original article indicates that it's around half of their total setup. Scott notes that they had a T3 (likely a frac T3) with 140 dial-in nodes, which aligns with the articles guess of 134 machines.

So I would say that almost definitely, they are using 1 (or 2 for some on the right side of the photo) external modem per PC connected to the 66 block, those analog phone blocks tied back to the channel bank/multiplexer, and the carrier's T3 tied in there.

No internal modems used at all.

And the person who posted the photo on twitter is none other than Scott Miller, founder of Apogee Software, publisher of some of the most revolutionary games of the late 80's and early 90's, and this BBS (Software Creations) was the cornerstone of distributing the shareware versions of those games. Very cool bit of history, I remember dialing in to Software Creations to download Commander Keen!