Just like I don't expect an building architect to pick up a hammer, I expect them to know enough about the process that they don't create something that can't be built or but can be built but for a few tweaks could be built 10x more efficiently.
Knowing the tools shouldn't elevate anyone to unicorn status. That's just the job.
This sounds sad to me. I would prefer a designer who would collaborate with engineering (who best know the constraints) rather than one who would guess what engineering can do (and therefore temper their designs). I guess it depends on the product you are working on, however. If you have a huge communication firewall between engineering and design (e.g. engineering is being done in India), I guess it is the only practical option that the designers be able to know what engineering can do.
There is a natural communication firewall when a designer knows nothing about code in the form of language and vocabulary.
The product teams, however, would often come back and say "this is too hard", and my job was to code something up in a few hours and say "no its not".
Designers embedded in engineering teams can also work out, but lose the peering benefits of working with other designers in a studio setting.
In my experience most of my work has cross-over between the two with a continuum of how much design skill is needed. For example in previous jobs we have had many cross-discipline roles like Technical Artists, Technical Designers, Gameplay Engineers and Rendering Engineers. Various art positions like Lighting Artists will even get in on the act writing shaders alongside engineers. Level Designers will very often be required to write scripts. In general the closer you are to the squishy human stuff the more design skill and knowledge its useful to have and visa versa. In my current role I'm engaged in experience design and engineering.
More generally it feels like there is a lot of gate-keeping between design and engineering. Neither is too keen on the other muscling in on their turf. I think this notion of the designer-engineer being a unicorn is mostly bunk promulgated to this end.
Designers having a working knowledge of programming and programmers having a working knowledge of design really does help collaboration and project resilience. Specialization will remain a thing but it should be looked at in shades of grey rather than black and white.
No offense to you or your friend, but you may be overstating the rarity of that type of person. The visual effects industry, for example, has an abundant supply of people that are simultaneously highly artistic and yet have deep technical knowledge.
I actually know a designer that does his own engineering. He's amazing, but he's one in a million. Kinda reminds me of Brian Eno.
What we need are better ways to collaborate.