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".
I mean, this doesn’t really change anything from a practical perspective. It just highlights that the verbage in the press release was alarmist.
It’s still a security concern that most users will be unaware of.