And manufacturers tend to lock down their Matter devices, too, so you can't flash Tasmota or ESPHome on them.
See: Shelly, Sonoff.
I was working on a Matter device but it never got certified due to high cost/lack of customer demand.
Matter states that firmware images “may be encrypted.” This is not a requirement, though encryption is allowed and may add security
(https://community.arm.com/arm-community-blogs/b/internet-of-...)
Disclaimer: I haven't tried serial flashing of Shelly/Sonoff Matter-enabled devices myself, just remember some complaints of customers that failed to re-flash such devices.