No, it isn't a limitation on the MCU1. There are plenty of MCU1 cars out there with 2021.40 software, even older ones with Autopilot 1. On Teslafi, over 30% of AP1 cars are on 2021.40 software.
It is a limitation on the configuration of MCU1 and FSD updates from AP2.5 to AP3 hardware. So few...