Yes, I saw them live on PPV. Even UFC 1. And I was studying BJJ with the first American to receive a black belt in (at the time) Gracie Jiu Jitsu in 1994. I'm well aware of what Royce did and how he did it. We were jumping up and down and screaming in my house when he went for the triangle on Severn when the announcer was oblivious to the submission attempt.
Royce was on the bottom for 15 straight minutes against Severn. Severn had no idea what to do in the guard, and Royce was a master at minimizing damage while he had someone in it. Severn also didn't have a clue how to defend a triangle, which is something any decent blue belt knows intimately.
Say Royce fought Nate, how would he win? Nate could box him all day, stuff his take down attempts (that leg stomp to the clinch stopped working for Royce about 1995), and if they hit the ground Nate would submit him. Royce was never great at BJJ, as evidenced by the fact that he never even won any competitions in Brazil, and the way he got choked unconscious by Wallid Ismail with a clock choke in a grappling match. Even when he won the UFC Royce couldn't last one minute against Rickson, Craig Kukuk (my instructor) was in Torrance for almost a decade studying and saw them roll countless times. Royce was not an elite Jiu Jitsu practitioner, but he was good enough that he could appear to be one going against guys who didn't understand BJJ. His Jiu Jitsu was better for combat than many of the guys who came out of Carlson's side of the family, at least defensively, because theirs was more of a top game with less reliance on the guard. So against a good wrestler a lot of Carlson's guys would struggle more battling for position - because their guard games were relatively weak compared to a guy like Royce who played that game all day. But those same guys would roll Royce, because they'd pass his guard and submit him.
And if you put Royce at any point in his life against some of the top guys today he'd get killed. There are a lot more elite black belts around now, because the success of the UFC created an entire generation of BJJ practitioners. And those guys are rolling against each other, Nate and Nick are rolling with Kron (Rickson's youngest son) all the time. For them, Royce's best day would seem like child's play because of the level of competition they're going against daily.
BTW, Kron went to 4 - 0 in his MMA career on New Year's Eve (in Japan, so a day earlier here), with a submission victory over Tatsuya Kawijiri. Go watch it on youtube if you want to see what high level combat Jiu Jitsu looks like, and remember that the guy he was fighting is vastly superior to anyone who entered UFC 1 - 4 (particularly 1). Kron's game is actually very similar to Royce's in that he relies on the basics rather than any crazy sport derived guard work. But Kron does it like Rickson, and it's on another level than Royce's ever was - which is why Kron can use it to submit the very best guys in the world in grappling matches.