It’s entirely possible to be a self-taught pianist. It’s also entirely possible to learn a second language without ever speaking to someone else in that language. What you can’t realistically do is become conversational, because you cannot practice that skill in isolation.

Similarly, it’s entirely possible to learn to develop software independently. You can learn languages and SQL normalization and good design all by yourself. You might learn faster with a teacher, but you can do it independently. You cannot learn to work well with others (properly subdividing designs, dependency ordering, giving good feedback, etc) in isolation, but honestly these are the things you generally learn on the job, because schools also suck at teaching these things.