"What does it really take to capsize a boat "
One good squall, that's all.
Consider the J/24 which was considered "uncapsizeable". Squall line came through the NY/NJ area in the 90's while a fleet was out racing, put one over on her ear, water came in through the cockpit lockers and flooded the boat out below. Capsized. And that's not the only one.
Push hard enough, and anything goes over. Most stuff also rolls back upright, if you have prepared the boat. That can mean sealing lazarettes off, or at least securing their covers, and putting in the companionway boards in rough wx.
The capsize ratios give you a way to compare you boat to others, and to see how much push it will take to start it over. But as long as you don't plant the mast in the mud, you can take reasonable steps to ensure it will keep rolling right over and come back up again.
Some boats have high "initial stability" meaning, you step aboard and they don't rock. Others are tender and have more "form stability" meaning, they'lleasily roll and put a shoulder to the water, and then dig in and remain stable at that heel angle. Most are one or the other these days, very few are actually tender AND tippy AND inclined to keep rolling on over.