I never heard of this guy, so I check his Wiki page.
Some excerpts:
Yeven Mezulah, the contemporary 17th-century chronicle by Nathan ben Moses Hannover, an eyewitness, states:
Wherever they found the szlachta, royal officials or Jews, they [Cossacks] killed them all, sparing neither women nor children. They pillaged the estates of the Jews and nobles, burned churches and killed their priests, leaving nothing whole. It was a rare individual in those days who had not soaked his hands in blood...
It seems Jews weren't uniquely targeted. Nobles, officials, and Christians were also massacred. Everyone.
And that's the story of history. Millions of people of every background were massacred, often simply because they were in the way.
As for the 300,000 estimate, that's based on old sources now discounted by modern Jewish scholars.
A 2003 study by Israeli demographer Shaul Stampfer of Hebrew University dedicated solely to the issue of Jewish casualties in the uprising concludes that 18,000–20,000 Jews died out of a total population of 40,000. He attributes many of these deaths to disease and famine.
Paul Robert Magocsi states that Jewish chroniclers of the 17th century "provide invariably inflated figures with respect to the loss of life among the Jewish population of Ukraine. The numbers range from 60,000–80,000 (Nathan Hannover) to 100,000 (Sabbatai Cohen), but that "[t]he Israeli scholars Shmuel Ettinger and Bernard D. Weinryb speak instead of the 'annihilation of tens of thousands of Jewish lives', and the Ukrainian-American historian Jaroslaw Pelenski narrows the number of Jewish deaths to between 6,000 and 14,000".
When I was in the Navy, I had a buddy that got kicked out of every bar we went into.