Well something responds somehow but not the expected thing. When I set my Arduino IDE to
and press “Burn Bootloader”, the following avrdude invocation is done
avrdude -CC:\Users\xxxx\Desktop\Programming_stuff\arduino-1.8.8\hardware\tools\avr/etc/avrdude.conf -v -patmega328p -cstk500 -PCOM1 -e -Ulock:w:0x3F:m -Uefuse:w:0xFD:m -Uhfuse:w:0xDA:m -Ulfuse:w:0xFF:m
And PIO does
avrdude -p atmega328p -e -C "C:\Users\Maxi\.platformio\packages\tool-avrdude\avrdude.conf" -c stk500v1 -PCOM4 -Ulock:w:0x3F:m -Uhfuse:w:0xDA:m -Ulfuse:w:0xFF:m -Uefuse:w:0xFD:m
so the only difference here is stk500 vs stk500v1 from what I can see. Try and change the upload_protocol in the platformio.ini to
upload_protocol = stk500
And retry. If that still does not work, I would need to know which exact external programmer you have connected to your board and what the Arduino IDE log for burning a bootloader says.