Spyware, insufficient resources, packet loss, viruses, and ISP issues aren't limited to dialup users. The things you mention as causing people to disco would cause anyone to disco, or at the very least have problems.
If HTC says a dialup connection is fine, then I have a hard time believing you know more about their game, and what it requires, than they do. You may well be more knowledgable about connectivity than I am, but the things you mention as specific issues for dialup users aren't specific to dialup users.
But none of this really matters. That guy got disco'ed, and you don't know why. That's all that really needed to be said.