I disagree with their interpretation of DSL as being interconnected. I'd say dial-up is a bit more nebulous, but also not quite "interconnected" in the way the rules are meant to prohibit.
The idea isn't to prevent linking between repeaters, it was to prevent a phone line from being hooked up to the repeater (i.e. an autopatch) where an unlicensed user could dial the phone number (intentionally or unintentionally) and begin making transmissions on the GMRS side. This is the reason for telephone interconnection being banned.
When you use DSL, you're using the phone line but your DSL modem is sending a digital signal on the phone line concurrent with any analog phone traffic. The phone company at the office end demodulates the signal and patches you into the Internet via their fiber trunks. At no point can someone dial your phone number and gain access to the repeater.
When you use dial-up it's a similar story except you're using the analog phone line to dial the remote office and send the data stream over the normal audio path. So yes, you're making a phone call over the phone line but someone can't just ring your phone number and gain access to the repeater. So even this, technically, should be allowed. I wouldn't test the FCC on it, but for technical reasons it should be permissible.
Their point about VoIP is spot-on though. A VoIP system like we use with Asterisk and app_rpt is basically a private PBX system. You're making phone calls between internal extensions, not connected to the Public Switched Telephone Network. There's no phone number for someone to dial, and no way for someone who isn't authenticated in the system from being able to activate a repeater. By restricting to GMRS licensees only (and keeping Ham operators off of the network), there is controlled access only to licensed individuals which meets the FCC's rules. Now if this VoIP system had a public phone number, it would be considered interconnected and would be in violation of the FCC rules. Again, someone could just ring that number and begin transmitting on a licensed service, which is what this aims to prevent in the first place.