1004
Undocumented Commands Found In Bluetooth Chip Manufactured in China Used By a Billion Devices.
(www.tarlogic.com)
This is a most excellent place for technology news and articles.
It ain't so.
To use the "backdoor" an attacker needs to have full access to the esp32 powered device already.
It's like claiming that being able to leave your desk without locking your PC is a backdoor in your OS.
Yes, this is about undocumented instructions found in the silicon but they are not executable unless the ESP32's firmware uses them. Firmware cannot be edited to use them unless you have an existing vulnerability such as physical access or insecure OTA in existing firmware (as far as researchers know).
It is good to question the "backdoor" allegations - maybe the instructions' microcode was buggy and they didn't want to release it.