I think the GP used the word "tuned" incorrectly / to make the wrong point here.

A general-purpose OS is one to which you can build a stack on top of it for any use-case you can think of, and it will cope with whatever stack you lay on it about equally well, because it hasn't been forced into a particular shape where it's much better at some things but much worse at other things. A "jack of all trades, master of none" OS.

Microsoft would call all consumer and server editions of Windows "general-purpose OSes." But Windows Datacenter Edition and Windows IoT Core would be non-general-purpose OSes — the former only exists to run hypervisors/SANs, and it doesn't support "stripping off" that layer, so if you used it for anything else, that layer would always be there, bloating things up; and the latter only exists to run on embedded devices, and it doesn't support "adding back" the extra frameworks and services regular Windows has, that would be required to use it for "more" than embedded use-cases.

An OS being "tuned" for a particular substrate (what the OS is good at running on), meanwhile, has nothing to do with the OS's use-case (what can be run well on the OS.)

An analogy: each mobile OEM's spin of Android only works on that OEM's own phones, because that OEM's phones have the required hardware wired to the right SoC pins, and the Android spin ships with a BSP that defines a device tree that matches that expected wiring. Thus, those OEM Android spins are "tuned for" those phones.

But in the end, they're all just Android phones, and they can all do the same things. All of these Android spins are "general-purpose OSes." They're all made to enable you to put any Android software you like on top of them, and run it just fine. (Contrast Android spins made by industrial vendors specifically for automotive or kiosk use-cases, where a given car company or kiosk manufacturer then produces a hardware-customized-and-tuned spin of that already-appliance-purposed spin. You wouldn't use a car-infotainment Android upstream for other use-cases; you'd have to undo all the car-infotainment stuff.)

Azure Linux is exactly like a phone-OEM "tuning" of Android (and unlike a vertical-specific Android spin.) Azure Linux is also like, for another example, the vendor-specific Linux "distros" [really, tunings] that ship as (usually binary-only) images for various Single-Board Computers.

In all three cases, a "tuned" fork of an OS is still intended to run anything a user might want to run on the platform the "tuned" fork was forked off of. It exposes a general-purpose surface to the developer — just one that happens to do some of the general-purpose things you ask it to do, more performantly than a non-"tuned" OS would on the same hardware/substrate.

And, in all three cases, the "tuned" fork accomplishes that by relying on device-specific knowledge and capabilities (i.e. drivers, device-tree entries, kernel patches, etc) that have been burned into the "tuned" fork rather than upstreamed. There's still a HAL between you and that stuff; your workload doesn't need to know the "tuned" fork has been tuned. It just benefits automatically, from the OS having a deeper understanding of the hardware/substrate.