Because serial ports (and modems) were pretty much the default thing to use (besides LocalTalk and AppleTalk) the desktop software usually supported that out of the box, no terminal required. This is also why Fax support was built in, because even then, any other type of peer connectivity wasn't really big enough. And around that same time, ClarisWorks was also really common on Macs.

Now, we don't know for sure that the author also had ClarisWorks, but we don't really know much about what was available in general (except the games, the sound files and the fax). But in general serial file transfers, modem and serial based methods were pretty 'normal' to have.