If I had to guess, the typical Arc user is a Mac user in tech. It doesn't run on Linux, most windows users wouldn't run it, and non-tech people haven't heard of it.
Then most engineering IC people will most likely run Firefox or Chrome, so you're probably looking at designers/founders/managers as your target.
Probably some interesting targets there, but not the type that the NSA cares about. Just pure conjecture on my part of course ;).
I've seen quite a few. In one of my clients's Slack there are at least a couple people advocating for it all the time. They're mostly DLs or in similar roles. I also know at least one developer who uses it.
I used it for a while for a very limited use case. Some interesting concepts. Mostly I found it annoying though. I also didn't like the sign-in thing but still wanted to experiment. I have dropped it altogether and kept Firefox as main browser (as it's been for many years) and Safari as a secondary. Both work much better overall for my needs.
The only person I ever saw using Arc was a designer at a tech startup, so this checks out.