Since I am the author of the howto mentioned above I can verify that it does actually work. (It doesn't need AvantFax however)
Brian, you are correct for the most part, however there are a few issues to note. Initially t38modem 1.0 cannot register on a remote SIP server, thus it cannot be used on any SIP Peer that requires authentication. (You need to use t38modem 1.2.0, which actually supports SIP registration, but fails in incoming faxes). Additionally I am not that sure that it can perform digest authentication as well, so it might not work for outgoing calls as well.
Also the use of an Asterisk server offers better control of the calls going in and out of the t38modems. Imagine a scenario where you need behind a single number to have 20-30 or even 50 t38modems answering a call, you need something in front to actually perform the allocations. Let alone that t38modem is a rather unstable piece of software (especially in it's 1.0 version). By having an asterisk in front you are able to retry a different modem if one fails to answer without hanging up the line.
Let alone things like changing the A-number/B-number depending on caller or destination called.
Well anyhow, it's a nice setup :)