I think if there was another "bridge" mac it would've been easier (setup LocalTalk over the serial port, could theoretically send the required TCP/IP libraries, to make it easier to go straight to a modern computer). I'd guess that the comment about "but no networking software installed." includes not having something like ZTerm installed already for direct serial communication, and also not having MacTCP or OpenTransport installed on the old mac (not to mention the missing hardware to set that up)

https://en.wikipedia.org/wiki/ZTerm https://lowendmac.com/1998/tcpip-over-localtalk/

I have a few old macs that I play with from time to time, and it's certainly possible to network them to modern computers, just becoming increasingly hard with the number of old software and hardware items you have to keep around to do it.