theory

Very unfair summary of Kohei's work imo.

MitA actually has tons of references to Capital, Gotha and various newspaper articles, in addition to exploring later unpublished writings such as letters and excerpt-notes. MEGA2 is mostly released online afaik; if you have the ability to read the various languages and abbreviations Marx uses in them you could easily check whether or not the sentences Saito selected are representative.

MitA argues against a Marxism based on constant expansion of production and consumption without environmental or sustanable concerns. Saito also doesnt argue that Marx never had such views; he very clearly argues for a progression with key turning points relating to Marx's research. He generally uses more well known and/or published works (e.g. the editions of Capital) to show how Marx's research, as seen in excerpt notes, is reflected in his writings.

I agree that it is a very academic work, but in Saito's defence he published a more popularised book in Japanese with similar themes (English translation coming soon iirc) so he is likely aware of the need to not just focus on super obscure marxology.