When it comes to Big Food (or Big Pharma, Big Insurance etc.), I'd prefer - at the very minimum - taking any claims against them at face value instead of dismissing them outright. Of course, follow it up with your own research.
Less resourceful people, like me, prefer a more common-sense approach of searching & reading multiple authors & articles to arrive at something they feel could be reasonably close to the truth.
Because Greenwald is in possession of a secret stash of documents leaked to him and only a few other people on the planet have access to those documents. Are you saying that Lustig has some secret data about fructose that only he knows about?
It's expert consensus vs a guy being loud.
Now don't say "conspiracy" because then you're just being faddist and contrarian.