Our "exact position" on many kilnforming matters may change over time, as we have full-time kilnforming researchers on staff exploring new questions, and re-examining old assumptions. I understand there is a lot of conflicting information out there about kilnforming methods. You are correct to question your information sources, and consider who has the most accurate and useful information for your particular application of the materials.
Our current position on the devitrification range is covered in TechNotes 4: Heat and Glass. The latest version is always downloadable from our website. http://www.bullseyeglass.com/education/
However, as Bert noted, the temperature is just one of many variables that likely combine to provoke devitrification. Glass chemistry, cleaning methods, and over-insulation of kilns (which can lengthen the time spent in the devit range) all can contribute to devitrification. If you want to understand what might be causing devitrification in your own work, feel free to start a new thread on the Forum, be ready to provide lots of details on your materials and methods, and Forum members and techs will help you investigate this.