In the picture with the phone cable plugged into the modem port directly next to it appears to be an Ethernet port. I know the blog post says there was "no networking software" installed, but the OS came with file sharing capabilities built in. Someone would have had to gone to effort to remove it. It might even have an old school FTP client.
That picture is of the thinkpad he used to receive the faxes. The mac doesn't have an ethernet port. I believe it would have had appletalk that maybe could have connected to ethernet using an adapter.
The problem with the Appletalk to Ethernet adapter is that would definitely need drivers that are unlikely to exist on the Mac.
Given that, then using the serial port with a dumb terminal would be the next best option, but IIRC MacOS didn't ship with a terminal application so there's a bit of a chicken and egg problem.
What might exist is an Appletalk implementation for the GPIO ports on a Raspberry Pi or similar board, which would let you use the built-in Appletalk file sharing to get stuff on and off of the machine. The retrocomputing scene has to have something like that, it would be extremely useful.
