All knowledge is self taught, because learning anything requires us to actively engage in the educational process.
Learning cannot be done to us or for us.
Our culture likes to confuse education with learning. The former is a certificate – a stamp of approval from an accredited institution. It's a way of branding ourselves to prospective employers.
Learning requires us to lean in and ask questions. It asks us to do more than simply memorize information – we must also synthesize that new information with what we already know.
So when someone says they're self-taught, maybe they don't have a relevant school transcript to point to. But they've read books, watched YouTube videos, done tutorials, asked questions in forums, failed over and over, and built up enough confidence in their knowledge to say, "I am capable of this."
You don't arrive at that point just by attending a class lecture.