Yes, Isaac Newton was an alchemist... in the mid-17th century. He died decades before Antoine Lavoisier founded "modern chemistry".

If Isaac Newton was an alchemist today, I think we would not spend much time listening to him.

Alchemy was a mix of practice and mystery back then. It's not the same as a modern alchemist, who is left only with the mystery, because modern chemistry claimed all the practice centuries ago.

It's easy to discount that when we have the benefit of 400 years of his work being verifiably true. Without that, the fact that he was an alchemist WOULD put a dent in his credibility, and you would be RIGHT in doubting the quality of his work. That doesn't mean you're excused from checking if it's correct, but having doubts about it's quality is perfectly reasonable.

Having beliefs that are out there doesn't prove that you're a crackpot, but it does slide the probability in that direction.

That said, a quick skim of the article doesn't reveal any glaring errors - it seems to be a good analysis.