It has no meaning whatsoever. Someone on the marketing side of the team decided that was a "competitive" number to present, outwards, and someone in engineering was tasked with, working backward from that number, coming up with some plausible calculation that resulted in it.
In the real world, they, like Azure and Amazon, will have single point in time outages that will wipe that out for a year or more.
Here is what an honest assessment looks like:[1]
"Historically (updated April, 2019) we have maintained 99.95% or better availability. It is typical for our storage arrays to have 100+ day uptimes and entire years have passed without interruption to particular offsite storage arrays."
...
"In the event of a conflict between data integrity and uptime, rsync.net will ALWAYS choose data integrity."