Yes, and no. DT's as used by these arm boards are basically reflections of how the linux kernel works on a given platform. A device driver developer needs to make a decision, say about how fast to program a clock divisor, so they then need a clock driver, and a device specific set of attributes that match 1:1 with the code paths in the Linux kernel which are making platform dependent decisions. When another OS comes along the attributes change because the driver model is different. This is visible in linux as its own model evolves. The systemready/ACPI specs tend to require more self describing buses (pci/usb/etc) where the devices are more fully encapsulated behind the bus, and things like power/clock management are either standardized by the bus interface, or via the standardized ACPI methods. AKA, putting an ACPI device into a low power sleep state is the same across every single device/platform because ACPI can notify some other firmware/management entity to perform the work rather than trying to fine grain manage the process.
So, your right, DT is a subset of ACPI, but its only really capable of HW description. So all these platforms (like the rpi) end up with proprietary ways of getting their management (the GPU in the case of the rpi) engine to perform platform actions. And that's needed because all these arm SoCs now have processors/etc that aren't visible to Linux because the model is no longer just about a central CPU doing all the work.
It's more like, the device tree describes the parameters needed to load and run Linux drivers for the hardware, given the exact git revision of Linux including the dts file. For one particular hardware variant/revision, of course.
So, your right, DT is a subset of ACPI, but its only really capable of HW description. So all these platforms (like the rpi) end up with proprietary ways of getting their management (the GPU in the case of the rpi) engine to perform platform actions. And that's needed because all these arm SoCs now have processors/etc that aren't visible to Linux because the model is no longer just about a central CPU doing all the work.