The CLA is Google's in this case. It gives them the right to distribute your copyrighted work. It also has some terms that say Google could sublicense and redistribute the jj codebase under another license -- which means they could for example release a copy of the jj code that was under MIT, or GPL2, MIT-Apache, etc. Or proprietary, even.
But the thing is, Jujutsu is under the Apache 2.0 license right now. You are within your rights to use Apache 2.0 in proprietary products. Literally anyone, at this exact moment, could clone the repo and integrate it into a proprietary product, or share proprietary binaries built on the code (rename it Gugutsu VCS) without giving up the source, as long as they obey the ASL 2.0 terms. The CLA really has nothing to do with this fact.
If you are personally worried about your personal copyrighted patches going into proprietary products -- or if that's your only worry -- you basically need to avoid contributing to all projects that are not using a copyleft license; just avoiding CLAs or whatever isn't enough. This has been true for a while.
If you are worried about a "rug pull" the next question is whether the developers can actually do that. That is a matter of social configuration of the project, not licenses. The thing with "rug pull OSS projects" is mostly about ownership and labor. Licenses and CLAs really don't matter much if 99.8% of development comes from one group of unified engineers. Such a setup is risky even with a 0% chance of any license changes. Their company might fail or they might all get fired. You didn't get rug pulled, but the project might die, which is basically just as bad.
But for Jujutsu, there is no majority of developers from any one company, and significant amount of code comes from the community, so the answer is more or less "no, we can't do that even if we wanted." The best we could do is start a whole fork and lose a significant chunk of the community and contributors. There isn't any one group of people with enough of an iron grip to do that without sustaining massive losses.
I think people sort of get "Can this become proprietary", with "Is some possible" -- in this case Jujutsu's code can absolutely "become" proprietary already in a real way. You can just go do that now. But a lot of "rug pull" things are prevented, I think, simply due to the fact we have a robust and strong community outside the (paid) developers
Ultimately, for a non-copyleft project, non-assigning CLAs like the one Jujutsu has are basically just annoying, legal ass covering mechanisms for Google's sake. I also do not like the CLA, it's just basically an irrelevant piece of paper as far as my principles on the matter are concerned. The Linux kernel DCO is basically just a CLA with fewer words, motivated by similar concerns to not get backstabbed due to the business logic of suing people.
If we got rid of the CLA for Jujutsu, I personally think it would probably still be nice to ask people to respect the DCO.