A developer can have negative performance, either by wasting time on the wrong thing, or by producing a solution that is flawed and needs to be rewritten (if the project survives). On the upside, a developer can invent a whole new way of doing things, and as such acheive things that an arbitrary amount of other developers wouldn't be able to. As such, 10x may be a useful metaphor, but in real terms is a gross understatement. It may make more sense in traditional enterprise settings where most things are determined from above, but in startup settings, a good developer can make or break the company.