Why would apple ever release an SBC product? I could understand if a community grows up around repurposing iPhone logic boards, but sourcing their laptop SoCs or boards wouldn't be feasible commercially.

If it's about developing the ARM ecosystem, the Pi boards have systemready certification with ACPI tables and all the creature comforts, with supply issues improving.

As a consumer, a hypothetical Zen4 APU SBC would perform better (7840u) on both performance-per-watt and absolute performance metrics compared to equivalent price M series, while giving you access to the mature x86 ecosystem.