The last release of OS X was 11 years ago but this site shows it having over twice as many users as macOS. It's totally implausible.
I run an unpublished service used by 1s of people and was hit by an influx of supposed OS X user agents that didn't correspond to my actual user base.
It appears that it's the Cloudflare Security Center scanning service? This would also explain how my unpublished service was located as I use Cloudflare for my DNS.