Throughput is actual throughput aggregated across all active interfaces. You can technically have eg. 8 x 10G interfaces assigned to your vMX but run it on a 1G license as long as you don't exceed the 1G throughput.
The 40G definition is "40G received and sent out" - matching 20G full-duplex.
From my knowledge discussing this with Juniper SE's it's still throughput-based even that you link states full-duplex. The capacity is based on how much traffic you send out egress. So yes a 1G license will give you 1G in- and outbound... but not 1G both ways at the same time.
I will ask again and revert with information - and ask to get the documentation changed if needed.