"Semantic" means making all distinctions you care about and not making any distinctions you do not care about. This means a custom notation for nearly every case. XML is such a tool. And XSLT is a key component to make all these notations compatible with each other.

That is not what "semantic web" means. Semantic web was a series of standards (rdf and friends) made by w3c from the early 2000s that didnt really catch on.

JSON-LD, ActivityPub, and scholarly metadata are all examples of Semantic Web technology that have caught on.

The GP asked "Why is RSS so special here?"

And XSLT in that context is interesting as one can ship the RSS file, the web browser renders it with XSLT to human readable and a smart browser can do smart things with it. All from the same file.