For the 10xer, it usually isn't that they do 10x more work, it's that they know how to make their org do 1/10th the work and get essentially the same results. They pay attention to business requirements and if you spot a senior engineer being listened to very closely by the people who make business requirements you've got reasonable odds of having found one.
The other type is the person who has an esoteric skill that lets you advance your business objectives by 10x-- for example, the optimization engineer who figures out how to cut server costs by 90% or whatever. These folks are usually viewed as wizards by other engineers. If you find an engineer being consistently handed all the 'crunchy' problems you've got a good candidate for this type, and again they aren't that rare.
Very, very rarely the two are the same person. If you ever work with one you'll probably know it pretty quickly, but odds are low.
To be fair, 10 people would write more code than a 10x engineer. But a 10x engineer can cut through complexity and deliver something simple that, by the virtue of its simplicity and brilliance in design, works (1) better, (2) faster, (3) has fewer bugs, (4) took 1/10th the time (5) requires less maintenance etc. etc.
maybe there are people who are 10x across the board, can't be ruled out.
Wait, is the “rockstar 10x engineer” a thing in real life? I get that people can be highly knowledgeable in a certain area or familiar with certain tools or highly effective at cutting to the core important details of a problem and making it clearer.