This post refutes the claim that researchers found a "backdoor" in ESP32 Bluetooth chips. What the researchers highlight (vendor-specific HCI commands to read & write controller memory) is a common design pattern found in other Bluetooth chips from other vendors as well, such as Broadcom, Cypress, and Texas Instruments. Vendor-specific commands in Bluetooth effectively constitute a "private API", and a company's choice to not publicly document their private API does not constitute a "backdoor".
Potato, potato…
Whether we call them ‘undocumented commands’ or a ‘backdoor’, the affect is more or less the same; a series of high-level commands not listed within the specs, preventing systems engineers/designers from planning around vulnerabilities and their potential for malicious use.
In that case, every stack that you use is riddled with those and we are all hosed. And yet somehow your computer, your phone and the internet keep on working most of the time.