Due to how the site takes into account ALL user-agent strings ever collected, it overestimates how unique an user-agent string is. Realistically in a given point in time, there are only a few dozen user-agent strings in widespread use (due to how few bits of information actually gets put into it). Unless you're using a special snowflake browser/operating system you should be fine.

I misspoke when I said it was simply User-Agent - they appear to fingerprinting based on other items such as installed fonts, etc. I believe when they say it's unique, it means, "unique". Not, "reasonably uncommon". And if that's the case, it's been up for years and has never encountered a system exactly like my current one. I'm on a very popular Linux distro used by most of my co-workers at a mid-size company, and I have the same set of work-related plugins installed as all of them, plus LastPass and Ad Block Pro. So not mainstream by any means, but also not going out of my way to be a snowflake, either.