QUESTION: To what degree can his descriptions be taken as "likely true" for any given (sufficiently resolving) system
ANSWER TO HOW “LIKELY TRUE?”
For all the good and valid reasons already laid out, it is a minimal to nil assurance that a blind purchase will also work in YOUR bespoke system in absence of a hands-on audition and experimentation in your bespoke system.
Simply put, it is a purely calculated risk purchase