Not sure how I feel about this.
Idk about SpaceX, but from a lot of the companies I see, a "Researcher" is a senior to very-senior level role that typically focuses on discovering vulnerabilities, potential new implementation, data analysis, etc.
A lot of time in engineering is spent on designing, implementing, testing, and maintaining things that are pretty much understood. To put it simply, most of engineering is doing things engineers already know how to do, at least in my experience.
There's not often much time to experiment outside of a few pre-determined solutions, and a lot of that comes down to tweaking the chosen solution to fit expectations. Most solutions aren't made by the engineers that use them, they are bought or licensed because it's just waaaay easier to make them work.
Researchers on the other hand from my understanding have the luxury to actually experiment and look for alternative solutions since their deliverables aren't based on profit but instead are based on results and data collection.