I'm surprised you need a source for this. Literally half of the server software out there refers to itself as "<thing> server".

Apache [1] "The Number One HTTP Server On The Internet". This is not referring to the machine hosting, it's referring to the software that you run to provide a service.

Postfix [2] "mail server"

IIS [3] "Web server"

I could go on...

[1] https://httpd.apache.org/

[2] http://www.postfix.org/

[3] https://www.iis.net/