Andy Ruiz Jr's career (simplified)

Indeed. He wasn't a 25-1 underdog for no reason. From an aesthetic standpoint at least, it was easily the biggest upset of all time.

 

You don't have to say "no mas" to quit, you just need to express to the referee through body language, overt or subtle, that you are unable to continue.

After the 3rd KD at the start of the 7th, AJ is looking for a way out. He gets knocked down, turns his back on the ref, walks to the ropes and lies back on them while taking his count. The referee tells AJ to get off the ropes. He walks back into the action again for a few seconds but without taking any significant shots, falls to the floor on one knee. This time he spits out his gumshield, gets up at the count of 8, again turns his back on the referee and walks to his corner to lie on the ropes, while the referee is telling him repeatedly to turn around. From the count of eight, AJ had spent another 15 seconds ignoring the referee and lying on the ropes, not walking forward when instructed to and offering only weak affirmations when the referee asked him "you ready to box?" The referee doesn't get any real indication that AJ wants to continue and waves the fight off, with AJ only offering a very subdued protest (compare this to Wilder's fulmination after Mark Breland threw the towel in, or Groves' response when Howard Foster prematurely stopped Froch-Groves 1).

Some of AJ's apologists will claim that all of this was an attempt to buy time but the problem is that AJ had been knocked down 3-4 times at this point and was on very thin ice. Turning your back to on the referee, lying on the ropes multiple times, ignoring instructions and spitting out your gumshield are seen by the ref as expressing that the boxer does not want to fight and is in no condition to continue.

AJ's quitting was more subtle than Dubois panicking, taking a knee to Joyce and having himself counted out but AJ quit nonetheless. He was hurt, gassed and had been knocked down 4 times by that point, the last time really pathetically. He believed he had no chance to win and was only going to end up getting knocked down another couple of times, before being totally incapacitated. The optics of quitting subtly were far less overtly awful than sprawling around the canvas attempting to get up or being sparked out cold. He was also saving himself additional physical and psychological punishment by quitting.

Another telling sign that he quit was how many fighters and ex-fighters implied or stated that AJ quit, even though it's typically taboo to seriously question a fighter's heart, especially at championship level.

 

Tyson had more hype and that was a bigger statistical upset: 42-1 vs 25-1 but aesthetically there's no comparison. Douglas was 6'3, 230 lbs and a naturally big man in shape, while Tyson was 5'10 at best and 220 lbs. If you knew nothing about boxing you'd think that Douglas was the favourite. AJ on the other hand was no less than 6'5, 245 lbs and built like the incredible hulk, while Ruiz is 6' at best and built like a sumo wrestler at 270 lbs. There is also the fact that Ruiz is a Castizo Mexican and Mexico had never produced a heavyweight champion before, so that made it look like an even more absurd mismatch to the casual eye. Then there's the nature of the fights themselves: sure Tyson gets beaten-up in every round save one but he only gets knocked down once and that's when he's counted out. AJ's defeat was, at least to the casual eye, even more decisive as he gets knocked down 4 times on route to quitting in the corner; at least Tyson tried his absolute best to get up and continue to fight even though he was completely out of it. AJ-Ruiz was an upset stranger than fiction.

 

Updated. Unsurprisingly Ruiz is still on a downward slope.