That’s the bootloader. The output is generated before the very first of your firmware is even executed. So by the principle of time travel not being possible, your firmware code can’t affect the output before it.
Of course, Espressif has an option to disable bootloader output, but that is by pulling / strapping GPIO15 low at boot, e.g. by using a strong pull-down resistor (like 1K) or a direct connection to GND, depending on what else you have connected to GPIO15.
