historical vs. equal temperament

[Orlando Fiol]

So, Jim, I'm quite intrigued by your pure fifths tuning, since, believe it
or not, I had never heard of it before.  How is it done?  Does one start
from a given point and just tune pure fifths all around?  Of course, that
couldn't be right, silly me, since it would give a rather wide fifth
somewhere around A-flat to E-flat.  so, what did you mean by the thirds not
being Pythagorean in a pure fifths temperament?

Thanks,
Orlando