Yes and no.

Orbitz, Travelocity, Hotwire, Expedia, Priceline and all the other discount airline websites buy their tickets from United and other carriers in bulk, allowing them to set their own prices while charging an arm and a leg for service fees.

Normally Hotwire and Priceline buy their tickets from United in L class (the lowest possible), so you can't make any changes once your reservations are ticketed. But if you're set on your dates and don't anticipate to make any changes to your reservations, they're normally the most discounted prices on the web.

The other websites generally allow changes...with the $100 change fee plus any fare difference.

So really, united.com's guarantee is only based on what the otherairlines.com sites are charging, not Travelocity, Orbitz, etc. etc. In the event that you do purchase a ticket through United and find that, say, Southwest or whoever else, has a lower price -- you do have 24 hours from time of purchase to get a full refund, according to their United Commitment.

OTW
-former United Airlines employee