It doesn’t have publicly-traded shares because it’s a private company, but it’s still correct to say someone has stock in a private company corporation (which isn’t relevant as Valve is unincorporated) that they own part or all of. Like with physical objects, they don’t stop existing just because they’re not for sale to the public. It’s an easy mistake to make, though, as the vast majority of the time people talk about stocks and shares it’s in the context of buying and selling publicly-traded stock.