The phrase "heel over" is particularly confusing. All sailboats heel on most points of sail. When he says the boat "heels over" at 10⁰.. I assume that means the boat's Angle of Vanishing Stability (AVS) was a mere 10⁰? If so, the boat was very unseaworthy indeed.
Seaworthy square riggers would have had an AVS of more than 70⁰ with closer to 90⁰ being ideal. Modern externally-ballasted keelboats have an AVS well in excess of 90⁰, many in the 120-130⁰ range, meaning the mast can be completely submerged and the boat will still come back up if it isn't down-flooded.
Another point that stuck out to me: He said that the additional gundeck in the 17th century lead to a change in tactics from boarding with marines and taking the enemy vessel as a prize to simply outright sinking of the vessel.
In fact, prize-taking remained the dominant goal of naval battle until the late 19th century when exploding ordinance and long-range standoff guns rendered it impractical. Consider the battle of Trafalgar in 1805: The British captured 21 French and Spanish vessels and sank only one.