His argument is based on the fact that Ptolemy's longitude coordinates are all tightly internally consistent but systematically divergent from real coordinates, and that the divergence is based on a misinterpretation of the zero-point as the Canary Islands, when in fact it was referring to the Lesser Antilles. Ptolemy describes the shape of the archipelago as being distributed North/South, which is almost perpendicular to how the Canaries are distributed but describes the Antilles well. His paper is here, he gets to the claim in question around page 13. He also has an hour long lecture on youtube which I haven't watched, here: https://www.youtube.com/watch?v=FzlF1eggBig
Anyway, how reasonably and seriously-taken is this claim?
Without having to check with any Classicists in any way whatsoever, after reading his paper, I can answer your question with: Not at all.
The paper appears to describe a reasoning process, however sketchy, by which Russo deduced the Lesser Antilles as the "zero point", by studying ancient sources. But looking at the way he presents his data, this is almost certainly not what he actually did. Instead, he most likely took the longitudes of Ptolemy, experimenting with various phase shifts and scaling factors until he found something that roughly corresponded to what he would have liked them to describe.
Then he went back to the texts to look for a justification of this, which is why there's no train of thought that can be followed throughout the paper - he's combing ancient authors for a way to justify his predetermined conclusion.
Finding a reasonable-looking way to fit or transform just about any set of decently-conditioned coordinates to some data set is something an undergrad in any number of disciplines can do using MATLAB over a lunch hour. I doubt, however, Russo has even that level of expertise, considering the ridiculous number of significant digits he cites.
But it takes a special kind of person to take whatever absurd conclusions the result implies and comb through manuscripts to find something, however far-fetched, to support it. For that I have to applaud him. However, I'm afraid there's no history here, just olympics-grade motivated reasoning.