I'm not familiar with that specific board or the charge circuitry in it, but it does, indeed, sound like it's designed for lithium (most embedded chargers are these days unless otherwise specified). You're also correct in that it can run off of any battery within voltage spec, but charging non-lithium batteries would not be recommended.

I wouldn't advise it unless you can disable the charging circuit when USB is plugged in. You may be able to put a diode inline with the battery to only allow current to flow out of the battery, but it would drop the voltage a bit. Still, may not be worth it.

If you really want to use the NiMH batteries, you'd probably be best to run them as a dedicated supply that doesn't go through the charge circuitry (i.e. don't use the battery connector).

More Info: https://rechargemybattery.com/can-i-use-li-ion-charger-to-charge-nimh-rechargeable-batteries/