I believe what Keppel is saying is that JtR shows an escalation and that if we backtrack we will find the picquerist before they gravitate towards mutilation. That the mutilations show evidence of also picquerism. That these can connect victims because it is a rare trait even among these serial offenders. Meaning Keppel has explained some of the wounds being a connection.
Martha Tabram seems to be a good example of a possible JtR candidate for reasons like this.
Bona fide canonical and then some.