Also interesting: If I just clone GitHub - lvgl/lv_port_esp32: LVGL ported to ESP32 including various display and touchpad drivers using
git clone --recursive https://github.com/lvgl/lv_port_esp32
cd lv_port_esp32
pio init -b esp32dev -O "framework=espidf"
echo [platformio] >> platformio.ini
echo src_dir = main >> platformio.ini
pio run -t menuconfig
on the commandline, I get
This working here 
Seems to have something to do with the folder structure, too (main vs src/).