But, as I mentioned above, I'm more concerned about how Aminet would interpret his statement.
We (Aminet) usually do not check licenses of sources uploaded more than twelve years ago when moderating a new upload - if you wouldn't have mentioned it, nobody (neither us nor the original author) would have noticed ;-)
But now that you did mention it, there's only one possible answer we can give you: If the author does not want you to distribute modified versions, we of course can't host a modified version. Trying to get in touch with the original author was the right thing to do.
while Aminet asserts that I am forbidden to upload any copyrighted material.
"Copyright" and "License" are two different things. Pretty much everything on Aminet is copyrighted, but all of it comes with a license that allowes free distribution (either by everybody, or at least by Aminet).