the easy thing to understand is: papers claiming to solve gravity cannonical quantization are few. In fact this is very likely the only one out there. (others would ditch QM and go full classical)
So if it was a scam, the scientific community would have a lot of resources to analyze and properly refute it.
However despite being ~ the only paper making this claim, and this claim being the main issue of quantum mechanics and a theory of everything, there is no excuse for the scientific community to ignore it.
But it has and keeps being ignored since the 90s. Even no forums except me mention it.
The thing is, yes this author make big claims. He is not a fraud, see e.g. his other papers. but he might make too high claim and his paper might have mistakes. At the same time, the scientific community is mediocre on average (much more than malevolent) and he has been wrongly ignored, like many other extremely potent results I know of.
I generally believe quantum research is extremely siloed and has NIH syndrome + the sunk cost fallacy. Quantum loop gravity and string theory aren't going anywhere and its been ~50 years since their formulation.

Independent researchers have a higher propensity of saying bullshit and also have a much higher propensity of thinking outside of the normative box and disrupt it, see e.g. physics research in the 20th century.
As a reminder Einstein refused peer review. I'm not advocating against peer review but it is not absolutely necessary to bring consideration to a paper.

Also, variations of classical gravity has already shown huge potential with MOND.
moreover it is so simple because his works build upon the work of giants such as Lorentz and Poincare.

> how do you explain the CMB?

The author has not to explained every subtopics of his theory though. He has an interesting result solving the whormhole information loss paradox though, which is huge.

Scientifically, there is very strong evidence to says that parkinson, alzheimer and cancer are mostly solved problems.
Nobody knows this because most of the scientific research is siloed.
The concept of meta-researcher, someone that actively research the research is almost inexistant, otherwise I wouldn't be the only person on the internet to cite thoses papers on major issues on forums (I have checked)
You have to understand that scientific papers in quantum mechanics have on average 0.93 citations. Now consider then most of the citations a paper get are from a future paper from the author himself, or from the same organization. I don't have numbers on that propensity but it seems credible to me that ~80% of papers have 0 citations outside of the organization/friends.
Does that means we have to discard ~80% of scientific knowledge ? No.
That means there is much more scientific revolutions to find in the past (even more for the pre internet era) that there is in the future. 1) because of the diminishing returns of the search space and 2) because most of the pearls have been consistently ignored. Some of the pearls are frauds or mistakes, and the rest are sleeping revolutions waiting to be woke by the few meta-researchers like me. Unfortunately, I do not have the media power to give them enough recognition so the best I get are the self fullfilling prophety of "nobody has analyzed it so I won't take my time to analyze it"

While (true) { /* do nothing */ }

More generally, I like to observe the reality that scientific research is "frozen". There is no public issue tracker for methodically tracking progress. Imagine a technical github-like repository (but with collaborative issue editing like on wikipedia) where we could observe in real time, with sub-threads, progress of the search space on major scientific problems. You could have an issue "implement quantum cannonical quantization" as a sub issue of quantum gravity and this link should receive major attention there. Except no, scientific research is not methodological progress driven globally, only miserably internally to some extent, by siloed organizations. Which is the reason mankind progress so slowly.
One day I will die and those things will not have changed much.