I haven't read the book, but this page is from their website and they still recommend sulfur if the TCEC is more than 10. Typically a clay soil has a high buffering capacity so it takes more amendments to change the pH up or down but a conventional lab soil pH test should tell you how much you need for your particular soil and if it is doable. Unless you're growing real acid soil lovers like rhodos or blueberries you shouldn't need to lower it too much from 7.1 if at all. If iron or manganese are low/unavailable enough for the plants that you grow, you would be seeing interveinal chlorosis on the youngest leaves of those susceptible.
https://growabundant.com/sulfu...
A plant tissue analysis including micros would tell you what the plants are able to get from the soil as opposed to what's actually in it.