I believe this is an unfounded assertion and a bit irresponsible to claim, considering the safety issue here.
It's pretty and it potentially provides greater range of motion. But it's still a ball resting in an open-bottomed hole a little bit smaller than the ball. And in fact it's worse because it's naked open to the elements as it is now.
Even if it were fully booted, you still couldn't claim its stronger, because there are no statistics of 300,000 vehicles running for 200,000 miles each to compare against.
Show me 1000, or even just 100 20 year old uniball 4runners (where the weight of the truck rests ON the ball), and show me that X% of the 20 year old uniball trucks failed vs Y% of the oem trucks, and that the X was significantly less than the Y, and THEN you can say it's "stronger"
100 20 year old uniballs on 4runners do not exist.