I found myself being really rigorous about specifications of everything. So this one thing came too much confusing. I can't find any proof of built-in complex type being subclass of numbers.Complex. This problem doesn't apply to other numeric types like int or float as they're given those confirmations.
Generally speaking, what should I do if I face deficiences like this one. I think it's expectable to meet them because of this document's quite narrative style. https://docs.python.org/3/