Non sense. Linux makes many details about computers much more explicit than Windows does. Even installing Ubuntu teaches you something about how computers boot, what partitions are and file systems do. Linux also makes it easy to get started in programming on the OS level -- thanks to the Internet it is way easier to find information about how everything works.

So from a learning experience perspective, a kid tinkering with Linux might gain more insight into how computers work and than, say, on Windows or Mac.

Agreed. I still remember my first foray into linux install. PC wouldn't boot afterwards and I had no one to shepherd me through it. These days if I want to get information on how to get something working, 9/10 someone already posted it for your benefit.

Some of things are worse, but some things got infinitely better.

That's today, but it wasn't so initially. You'd learn about your monitor's scan modes, how the disk is structured, dumping bytes between devices, optimizing for load by placing bytes on the outer sectors of floppy disks, using RAM as a disk to speed things up, and whole lot of "useless" things that build machine sympathy.

Yes, but it's also true that drivers and peripherals are generally a lot more complicated today and hardware services more federated across layers of dependency and security than in the days of DOS or Windows 3. Reverse engineering today's Windows 10 wifi-based encrypted RAID disk array is a good deal trickier than a 30 year-old DOS FAT IDE drive ever was.

Well you still have easier and more difficult distros. You can go easy route and play with Ubuntu or Fedora but you can approach linux from the hard side and try Arch or Gentoo