>people saying that the fan "will always draw 0.3A regardless of the voltage,"

That's nonsense. 0.3A is the nominal current. Lower voltage=lower current. It could be a little higher if airflow is impeded.

>Surely a 12V/0.3A rating would mean that the fan has a resistance of 40 Ohms and the current would just be I = V / R.

Ohm's law applies to DC in resistors. Fans use coils which have inductance as well as resistance. They commutate the DC into AC for the coils. Plus most DC fans are brushless and you can't measure the resistance directly because of their electronic commutation.