ARM isn't nearly as interesting given the strides both Intel and AMD have made with low power cores.
Any scenario where SoundWave makes sense, using Zen-LP cores align better for AMD.
AMD does not have any product that can compete with Intel's N-series or industrial Atom CPUs, which are designed for power consumptions of 6 W or of 10 W and AMD never had any Zen CPU for this power range.
If the rumors about this "Sound Wave" are true, then AMD will finally begin to compete again in this range of TDP, a market that they have abandoned many years ago (since the AMD Jaguar and Puma CPUs), because all their resources were focused on designing Zen CPUs for higher TDPs.
For cheap and low-power CPUs, the expensive x86-64 instruction decoder may matter, unlike for bigger CPUs, so choosing the Aarch64 ISA may be the right decision.
Zen compact cores provide the best energy efficiency for laptops and servers, especially for computation-intensive tasks, but they are not appropriate for cheap low-power devices whose computational throughput is less important than other features. Zen compact cores are big in comparison with ARM Cortex-X4, Intel Darkmont or Qualcomm cores and their higher performance is not important for cheap low-power devices.
A cursory search shows that the AMD APU used in the Valve Steam Deck draws 3-15W. Limiting the TDP to 6W on a Steam Deck is fine for Linux in desktop mode.
It is not a device that AMD sells on the open market, so it does not compete with the ubiquitous Intel N-series CPUs or with the Arm-based CPUs from various vendors.
Like I have said, since Jaguar and Puma, which are older than the first Zen, AMD has never sold on the open market any CPU/APU designed for a TDP of 10 W or less.
While for some AMD APUs, like Ryzen Z1, which are designed for a TDP of 15 W, their specification says that they have a TDP that is configurable down to 9 W, when such CPUs are configured for a lower TDP than they are optimized for, they become inefficient, by having a bigger die area, i.e. a higher cost, and a lower energy efficiency, in comparison with the CPUs that have been specifically designed for that lower power.
>> AMD does not have any product that can compete with Intel's N-series or industrial Atom CPUs, which are designed for power consumptions of 6 W or of 10 W and AMD never had any Zen CPU for this power range