A home lab was generally seen as something to experiment with and learn something from. Setting up a Plex server is literally installing one piece of software on anything newer than a Core 2 Duo.
I started my own homelab like this, and still learned from day 1.
It grew over time to serve many purposes, has seen many stopped/failed attempts, has had many lives (I mean, recreate from scratch to learn X, Y or Z).
There's not a particular day or a particular addition or a particular level of complexity when it became a homelab, I think of it as such from day 1.
