… but YAML ain't a markup language.

(That's the 'A', in YAML, literally.)

That's a backronym; originally it meant "yet another".

The current acronym is old enough to drink. The original acronym, AFAICT looking through The Wayback Machine, lasted less than a year. (The original language was pretty wildly different, and much closer to a markup language.)

For those that don't want to wait for the time capsule, here is the earliest example of it (I've shortened it from the original for brevity; best I can gather this should also have been valid):

  <timesheet.clarkevans.com:record>
    <person:record>
      <id:int>
        293945</id:int>
      <name:record>
        <given>
          Clark</given>
        <family>
          Evans</family></name:record></person:record>
  </timesheet.clarkevans.com:record>

You can feel the XML influence.

Oddly it kept the "Another" well into the evolution of not being a markup language anymore, but on the grand scale of things, didn't really keep it that long.