Development is sadly the job that is becoming the kitchen sink of "You ought to have skill X" where X is anything from sysadmin, computer science, business, security, product, bare-metal, networking, infrastructure, management, statistics, operating systems, math, social, and industry knowledge.
If you aggregate all of the "Every developer should know X" posts and blogs, the list would probably be very long. It only promotes shallow signaling instead of actual competence (I only need to know enough about X to make people think I know about X).
Meanwhile, your salary will still only compensate you for one skill set: software development.
Product managers, especially technical ones, can bridge that gap without being too deep in either skillset. A PM, on the other hand, isn't expected to know all of the nuances and gotchas or frameworks of whatever language the devs are working in.
Devs will slowly learn the relevant knowledge anyway, just at a slower pace than the immediate needs. And yes, after a certain number of years, that dev could be good at both. But you can't only hire devs good in both places for every position at every company.
I haven't really looked, but I can't think of posts of the form 'Every manager should know X'. From my lack of data, I pose that it's interesting how management is seen as a transferable skill, but software development isn't.
If you aggregate all of the "Every developer should know X" posts and blogs, the list would probably be very long. It only promotes shallow signaling instead of actual competence (I only need to know enough about X to make people think I know about X).
Meanwhile, your salary will still only compensate you for one skill set: software development.