Oh wow this really is operating-system specific regarding the upload_port when the value is usb. PlatformIO performs a sanity-check whether that “file” is there before uploading.
Please add
board_upload.require_upload_port = no
to the platformio.ini.
However then -Pusb doesn’t show up in the avrdude invocation anymore – but it still seems to look for the right thing. Worth a try. In the meantime I’ll raise an issue.