To begin with, I should say that I am not really familiar with the Newton. However, the procedure described under the link in your message appears to be similar to the one in
KB article # 22229 (for Mac-to-Mac transfers). Both are not TCP/IP; rather plain communications operations modem to modem.
Two randomly chosen modems, directly connected to each other (that is, a local connection), may have a problem negotiating. In such a case, a very simple line simulator (providing the necessary current) can be built from a 9 V battery, a battery connector, a 330 Ohm resistor and a slightly modified RJ-11 to RJ-11 cable. Do NOT use any line simulator when a modem is connected to the public telephone network.
The Mac would need some kind of terminal emulator with file transfer capabilities (such as the shareware program
ZTerm).
Jan