I investigated more and it seems that one can indeed perform NAT with Linux netfilter without the Masquerade action. If one knows the address of the interface, simply using the “SNAT” action with a to-address of the outbound interface will achieve the same result as using the “MASQUERADE” action, as long as the address of the outbound interface does not change.
But, this fact only matters for the actual underlying netfilter. I should have been thinking about OP’s application specifically. For OpenWRT it probably does just mean Checked->NAT, Unchecked->No NAT.