Dr. Hartlieb's alleged involvement in the creation of the Voynich Manuscript

[Oscroft]

  (2) Dr. Hartlieb was the only person at the time who possessed the full spectrum of knowledge covered in the Voynich Manuscript. 

I confess I'd never heard of Dr. Hartlieb until today. But how could you possibly know he's the *only* one?

Going from something along the lines of "I don't know anyone else who had sufficient knowledge apart from Dr. Hartlieb" to "Dr. Hartlieb was the only person to have sufficient knowledge" seems like an example of the Holmesian fallacy, if that's your reasoning.

Why couldn't other people you haven't heard of have sufficient knowledge?

(And as an aside, while we still can't read the VMS, how can you even know what full spectrum of knowledge was needed anyway?)

Not devastated,
Alan