Where do you even get the “media” part in it?

I’ll quote myself because I already answered that: « Media is the plural of medium, which means “intermediary” ». That’s the textbook definition of the word media, and the consensus about what it means.

Whatsapp is the middleman/platform, thus intermediary (= “medium/media”), between two people socializing. I don’t understand what you don’t understand.

A social network is social media that allows to form groups or communities. Two people communicating is not a network, the whole point of a network is interconnection between members of a group.