Thanks for the clarification.
I mistook VIN for the 5V path.
If a battery is connected to 5V (I’ve seen some people doing that on some video),
if the USB is plugged,
the device will actually draw power from whatever has higher voltage.
If you use a 3.7V battery, the device will draw power from USB and the battery won’t discharge.
But yes a diode would be a good advise,
to prevent damage to the battery in case it’s not a rechargeable battery
or even if it’s a rechargeable battery because there would be no connection to the charger controller.
So, I guess bottom line is that a rechargeable battery should be connected to Bat+ only
and a non rechargeable battery could be connected to 5V pin and adding a diode.