This person has bad ideas (going further, this person is synonymous with bad ideas and therefore only has bad ideas).
> I won't say crack pot because he is a legit tenured astronomer at Harvard, but I have seen his papers roundly critiqued on the merits at too many journal clubs to count
Some of his ideas have been proven to be bad although he has an expert understanding of the subject.
> Its a bit outrageous to claim he's Galileo ... against the weakness, and untestability, of his ideas.
This person is making a spurious analogy because Galileo's idea was a good idea and his ideas are bad ideas (some of which have been proven to be bad ideas).
It is ad-hominem in nature because GP is disposing of the content of the article by claiming the above analogy is spurious by pre-supposing that Loeb only has bad ideas.
There is nothing to be read here about the reaction of the scientific community to unconventional ideas which is the content of the article, especially if you pre-suppose that Avi Leob only has bad ideas and therefore was rightfully dismissed by the scientific community (which is what the GP is doing).
If the argument was in fact about whether or not Loeb's ideas are being unfairly dismissed (or in the weaker case, rightfully receiving pushback), that would directly be talking about the content of the article and generate a good discussion. (In fact I disagree with what Loeb is saying in the article and have posted a comment as such below)