That’s on me, I’ve been playing my tavern brawler for too long and overlooked that most people don’t have imrpovised weapon proficiency. It looks like using most ranged weapons in melee is maybe improvised for two reasons? Like, the ammunition property makes it improvised, but also the “ranged weapon to make a melee attack” rule makes it improvised. Which I guess lines up if you take it as Javelins being good for melee and throwing, while darts are only really good for throwing- makes sense to me, although it’s weird to have the same thing said twice over (a ranged weapon is improvised, but also an ammunition traited weapon is improvised, and only ranged weapons have that trait so they’re already improvised in melee)
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