Yes, that is pretty much standard behaviour in the TinyMCE instance in Optimizely CMS.

Whenever you enter a link (usually in External Link, but I guess also in Site Relative Link, as in your example) that matches a local page, Optimizely will change that to a Page link. Optimizely favours referencing content IDs whenever possible, so it makes sense to reference the Content ID and not the actual URL/URL segment (since page location may change but the Content ID does not). 

Maybe "Site Relative Link" is for a custom "redirect", because if Optimizely is going to change that to a Page Link I don't get the point of it.

As Arild says, converting the relative URLs to Page Links is default functionality. This is what keeps the links from being broken, if you move a page and thus updating it's URL.

Have you performed step 3 in the documentation you linked to? It should alter this default behaviour.

3. To make relative links work in TinyMCE, set the convert_urls setting to false to prevent TinyMCE from converting the value.