Apple says this about that:"You can use SF Symbols in apps running in iOS 13 and later, watchOS 6 and later, and tvOS 13 and later."
The way I understand this is that it refers to the technical possibility of using it through XCode, which is only possible since iOS 13. Otherwise I'm pretty sure they would have phrased it as "may" instead of "can". But since I'm not sure I asked the initial question and would highly appreciate if someone from/close-to Apple could elaborate on this.
It would be puzzeling to me if this wasn't legal because the entire "Creating Custom Symbols" thing would also not be covered. Let's say I export a template and change only one pixel, then import my "Custom Symbol" and use it in my iOS 12 app.