What's actually happening here is that many people now erroneously use the word "design" to mean "visual design in the tradition of waterfall-centric, industrial graphic design where ideation and production are separate," which in the context of software is extremely ambiguous, anyway, since it could refer to illustration, user experience design, graphic design, information architecture, and plenty of other things.
Yes, there are specializations. But the stereotypes of technologically illiterate "designers" and neckbeard, tasteless "developers" are outdated, though unfortunately still pervasive in the industry. There's no reason that anyone identifying as a "designer" can't gain a deep understanding of modern web or native development technologies and environments. There's no reason that someone identifying as a "developer" can't learn about "design" and the creative process.