Yanmar, AFAIU, is a big maker of various industrial and agricultural engines, so I guess the Yanmar marine engines are variants of those.
For Volvo Penta, I'm quite sure the bigger ones are marinized variants of Volvo car and truck engines, but for the smaller ones, they might sell them also as gensets or such, not sure.
Beyond Yanmar and Volvo Penta, there's a plethora of engine makers (Beta, Nanni, Westerbeke, etc. etc.), which make marinized versions of Kubota engines, which are AFAIU mostly used for agricultural and industrial equipment (e.g. those ubiquitous small Kubota tractors). These are generally well regarded, and sell for considerably less than Yanmar and Volvo Penta.
As another commenter suggested, I read the assertion about "the largest marine engine manufacturers use mass-produced automotive engines" as "largest engine" not as "largest manufacturer". Maybe the author intended the latter.
So likely both you and the article author are correct.