Without knowing the people involved, there's not much you can say about what happened (or even if it did happen ... nameless claims are always somewhat suspect, no disrespect intended). Unnamed people from style A are always beating unnamed experts from style B. This usually happens to unnamed people both ways. Unnamed judo black belts tap unnamed BJJ black belts, and unnamed judo black belts are tapped by BJJ white belts. The cool thing about them all being unnamed is that no one can verify or contradict it.
Most judo BB's wouldn't fare well at all on the ground with a BJJ BB, to put it mildly (though some could either stay on their feet or regain their feet instantly, much as is seen in MMA when BJJ'ers without good takedown skills discover that they've no way to get the fight to the ground).
But like anything there are exceptions. Gracie Magazine reported awhile back that Kenzo Nakamura and Roger Gracie rolled hard on the ground for 10 minutes without either tapping, though from what I've heard Roger would definitely have won on points if they were counting. If he can last 10 minutes against Roger going hard, presumably Nakamura would be a handful for your average BJJ BB.
Of course, Nakamura won Olympic gold in 96, and is a world class athlete, apparently using his athleticism to keep Roger from submitting him. Your average BJJ blackbelt (and there are over 5000 of them now) is going to find that level of athleticism fairly hard to deal with. Of course Nakamura would (and has in the past) go through most judo BB's very quickly as well, there's not much point for most judo BB's to look at Nakamura and say 'that proves I can hang with a BJJ blackbelt' ... guys like Nakamura are freaks, as anyone who's worked with them (judo or BJJ or wrestling or anything). Good examples to prove there are always exceptions, but not very useful for comparison otherwise.
Rhadi Ferguson (BJJ black belt and judo Olympian) has written that most Olympic level judoka are just BJJ blue to purple in ground skills, but would actually do pretty well against most BJJ black belts (though not the elite) on simple athleticism. People always discount just how much difference that can make - watching Sapp vs Big Nog's fight is a good reminder of a white belt with nothing but huge athleticism giving a black belt a rough time.
As well, you have the occasional freak like Penn tapping recreational judo black belts in documented (youtube anyway) videos - once again, most of us aren't Penn, and what he can do, like what Nakamura can do, has little to do with what we can expect from most of the population.
