Yep, you need either MH Mag's Towing Guide for your specific year, OR the Owner's Manual for your specific vehicle. We were toad shopping in 2006 and chose Corolla because it didn't have restrictions. I do know that Toyota moved the xB to a different platform over the years, and that could affect the recommendation.
Sounds to me like the manual transaxle doesn't rotate in a way that splashes oil around while it's being towed in neutral. So they want you to stop once in awhile and give the bearings a fresh oil bath.
The MH Mag list is only those vehicles which the Manufacturer says are towable and under what conditions they approve it. So barring clerical errors, you should be able to rely on MH Mag as you would rely on an owner's manual page.