The other problem with the new rites is admitted by Fr Hesse, when he said his ordination was done "by the book". This implies that many are not.
Yes, and OTOH, it also shows the mind of the SSPX that when done "by the book," the ordination is valid.
This has been the case since the start of this crisis. In my mind, most (many?) NO priests are invalid due to the consecrating bishops not going "by the book."
A few decades ago, the SSPX had a type of running list of these bishops, as well as those known to always go "by the book." So when a NO priest came to them, the whole investigation might only be a matter of asking who ordained him. As was the case with Fr. Hesse and who knows how many others.