Yeah, those competitions are not really representative of actual skill.

I was into competitive math as a teenager and was somewhat successful, but I actually kind of suck at math.

Similarly, I'm a professional developer but I'm really bad at competitive programming: what usually happens is that I know how to solve the problems but the time limit is too low (for me, at least).

I'd say success in competitions is a good indicator of dedication and perseverance, but not sufficient to spot someone who's good at the job.