Aliens are as likely an explanation as anything found so far. Given that no other examples of this language have been found it's hard to imagine it could be any real human one.
I think it's very easy to imagine that a real human wrote this book, since it's right in line with the materials available in the period. It's not written with some futuristic material like say... laser printer toner. Just because we can't translate something (which may just be gibberish) that doesn't prove the existence of aliens.