I’m not the person you asked, but I do.
> the proper emdash—a double long dash with no spaces around it
The spaces around it depend on style guide, it is not universal that they should not exist.
> That's because most keyboards don't have an emdash key
Nor do they have keys for proper quotes and apostrophes or interrobangs, yet it doesn’t stop people from using them. The keys don’t need to exist.
> That's what makes it such a good giveaway.
It’s not. It might be one signal but it is far from sufficient.
> I'm happy to be told that I'm wrong, and that you do actually use the proper double long dash in your writing
I do use the proper em-dash in my writing—and many other characters too—and my HN history is ample proof. I explained at length in another comment how I insert the characters, plus how simple it is if you use any Apple OS.