The Book Switch Theory

[ReneZ]

and there are reports that he even boasted about it.

Of course, if we assume that we cannot believe a single word Voynich ever said, then we cannot also believe any of his boasting.
In fact (more seriously), his various boasts should be among the first thing we should treat with a grain of salt. I certainly do. That also applies to 'the other theory'.

All of this is not too important for a book plot.