Yup. You can go to some gyms where they basically give out belts for nothing. You can be a black belt there and go against a purple belt in a really good gym and get smashed.
More than likely it's because he doesn't train in a gi much so his "gi jiu-jitsu" is at a purple belt level--gaining a belt in BJJ means being proficient in both a gi and without a gi and both are equally important in their specific sphere of use (although it is not uncommon for a sport fighter to focus on nogi since competition is his only goal)--on a side note the saying also goes that there are "black belts" and then there are "BLACK BELTS," and the same goes for purples, it depends on a number of factors. Purple is, or at least should be, the gateway to the elite ranks of grappling as well--thus a purple could be just on that cusp and still wilt against black belt level competition, or a purple could be a master grappler capable of holding his own with almost anyone, but who is still missing a few details in some areas of his game or needs to round out their skills in some way to be a true master of jiu-jitsu.
The gi does change things a lot (in real life people are likely to have clothing on and if you know how to manipulate them with that clothing you are at a huge advantage, not all martial arts technique is about being locked in a one-on-one scenario with safe surroundings either), and a winter coat is basically a gi--being a black belt means having mastered ALL the concepts of BJJ, not just the MMA or competition aspects, which are mostly effective in a sport fight where two guys have already stripped down to their bare minimum clothing and are in a safe, secure environment with no surprises coming etc.
Who you can beat is much less important than your ability to grapple using both the technical details and the strategies of BJJ as well (although if you have mastered the strategies etc you should be able to top most people ranked below you). JLau tends to gas, which means he has still not mastered one of the core principals of BJJ--maximum efficiency of energy output.
Given some training a good athlete might still be able to overpower his foe and maybe even submit him, but that is not the same accomplishment as a mediocre athlete who can subdue his foe simply by out maneuvering him through technique, all without wasting energy or taking unnecessary risks. Most of the top "black belt" level bjj guys are excellent athletes AND masters of efficiency when they grapple. Without the gi it is much easier to get away with being the better athlete in general but in a real fight that's fairly uncommon--the gi provides additional control and leverage and the ability to use it is TANTAMOUNT to being a legitimate black belt.