I was asking about your estimate. And Circana is only about US data. It wouldn't account for non-US sales.
Anyway, according to Steam, Starfield generated $235 million in revenue.
If we divide $235 million by $70, that's 3.35 million copies on Steam.
But not every region has the game for $70. I could buy it for $40, for example. So let's take the average price of $60 per copy, which puts the total number of copies sold around 3.91 million copies. Or 4 million as I said.
Now Helldivers 2 has broken Starfield's CCU count. If Starfield sold 4 million copies on Steam, it is safe to assume that Helldivers 2 also sold 4 million copies.
I was being conservative when I said 3 million.