I'm an engineer (mostly web) and I am very tech savvy, and extremely wary on the internet of scams. However, if this site had to come me via a trusted channel, I would have fallen for it maybe 80% of the time. I hardly ever login to steam as it's always running and while I have 2FA my password would have been stolen in this attack for sure. I don't think teenagers and non-techy users would stand a chance against this…
edit: Image in question: https://i.imgur.com/hVTEKfD.png