Old chips cost Apple more money now not less. The A15 and 64GB NAND probably have low volumes and cost Apple more money going forward. So apple likely just upgraded to what the supply chain can reliably do for a while. Prices might go up even in six months to a year. So instead of buying a 200$ bumped to 250$ A15 Apple TV in six months they are probably upgrading it to make sure future price bumps remain relatively low and justifiable for upgraded hardware.

Very interested to know what TV manufacturers are doing who have heavily advertised their Android UIs which are bread and butter of collecting data. TV panels themselves have no reason to have price increases in memory shortage.