Ok, don't panic.
Two possible explanations here.
1) The guide is referring to an "expansion only" game; an option that HoW gives in the opening menu, for players who are sick of the main game and just want to play the new areas. The expansion areas have a minimum level limitation - I think it's minimum average level of 8, or 9. So if the difficulty is structured for lvl 8 characters, the lvl 1 characters would obviously have a hard time. Further, the first fighting area (Burial Isle) of HoW is a notoriously difficult area for inexperienced players.
2) Second option: the guide is referring to
HoF mode. It stands for Heart of Fury mode - a difficulty level (toggled on or off in the Configuration menu) that is miles above "Insane". It was designed for experienced characters who have been through the game already and gamers who want a challenge. Apparently it is beatable from scratch (Dundee Slaytern has done it, with a two-character party), but it requires much patience and reloading. Monsters have more than three times their normal HP, and Thac0 and AC to match. Stick with a relatively normal difficulty level until you and your characters are fully ready to ramp it up.
Go ahead and roll up your characters, Cary. You can install HoW at any point in the game.