Ubuntu is used by seven percent of web sites, not servers. The journalist does not seem to be aware of the difference.
I suspect Linux powers a much higher proportion of servers than of sites, because so many big shared hosting companies use BSD, which means it is used by LOTS of sites on each server - which is why Unix shows at 64% in this survey.
On the other hand Linux is often used on dedicated servers, VMs which tends to mean one server install per site, or per site owner. Linux is also popular for shared hosting that lets you run your own web server instance.
One interesting thing that the article did note was the relative youth of Ubuntu. I installed it several years back to 'dive into linux' and just found it simpler to run a fresh install of ubuntu on servers too.
> What advantages does Ubuntu have over Debian on a server?
I don't think there are huge differences between the two tbf.