Looks like I was dead wrong here- turns out there’s another JC tweet that says: “If you use a weapon in a way that turns it into an improvised weapon—such as smacking someone with a bow—that weapon has none of its regular properties, unless the DM rules otherwise.” So bonking people with a crossbow wouldn’t count for GWM because the crossbow isn’t heavy when you’re not shooting it
After their efforts with 4e’s VTT, I’m not surprised they’re so happy to be able to show basic functionality