I do not agree with your point, for various reasons already elaborated upon in the comments section here.
Regardless of that, I find it very puzzling why you don't simply email the authors with your critique, instead of writing a pretty thorough comment on HN that they will likely not read? That would be far more constructive IMHO.
Keep in mind that this is a preprint. From my experiences in this field, preprints are usually very sloppy and are simply a means to lay "first claim" to a discovery. There are usually a ton of typos/other issues to be sorted out, or even worse, bugs in many key proofs - I am not saying this paper is guilty of this, just that expectations should be low regarding exposition quality in preprints.
Regardless of that, I find it very puzzling why you don't simply email the authors with your critique, instead of writing a pretty thorough comment on HN that they will likely not read? That would be far more constructive IMHO.
Keep in mind that this is a preprint. From my experiences in this field, preprints are usually very sloppy and are simply a means to lay "first claim" to a discovery. There are usually a ton of typos/other issues to be sorted out, or even worse, bugs in many key proofs - I am not saying this paper is guilty of this, just that expectations should be low regarding exposition quality in preprints.