What are you talking about?
This is the exact "secret Axis modifier".
Whether the multiplier lies on to the unit or weapon itself is not irrelevant to the topic. It's just tomato to-ma-to. The important thing is that AT Satchel does deal 20% less to the Axis infantry while it can wipeout Allied infantry.
Link only stats W-Germany because it doesn't have enough space. I have confirmed with CoH2 tool that satchel indeed deals 20% less damage to Axis infantry.
dont bother interacting with vipper, dude.