Do we know that they don't? A dedicated ASIC would probably be spotted by some random person doing a teardown. But a DSP chip doubling as crypto miner when not in use, that could easily escape post-purchase hardware inspection.

(Might be easier to discover it on the design/procurement/supply chain side. Perhaps stock players would find out when investigating their investments. A high-profile brand putting crypto miners on consumer hardware is newsworthy, and news move stocks, so there's incentive to leak the story.)