Is this correct? “All intercompany transactions related to the inventory transfer must be in the base currency of the purchasing subsidiary (destination).”

correct. When you create a PO you select your currency; everything from there on out is in that currency

But it says base currency of the purchasing subsidiary. The PO could be of any currency. It should not be limited to the base currency although I think it's not true anyway. I can enter an intercompany in a foreign currency based on my testing.

sure, it is not limited to just the base currency. but all the transactions are limited to the PO currency, that’s where you select it