Your cow-orker seems like a workaholic. Probably felt inadequate and tried to makeup for his feelings of inadequacies by putting in extra hours.
Especially in high tech, people have different skills. For instance there are a set of people who struggle to comprehend subject A, whilst others just see it. Conversely the latter might struggle with B, whilst the former see that clearly.
I worked in an area where you needed hardware and software skills. Some people were excellent at the hardware, but were totally lost when it came to software. But one really couldn’t be successful if you didn’t have both skill sets. Because hey, no one measures with a hand held meter, you attach probes and collect data all night. Then you ftp the data to a server, then write some scripts to reduce the data, then you could do an analysis on margin to spec. If you struggled with any of these steps, you were late, and we need to find someone who can get the job done . . . someone’s getting a bad review.