It seems unlikely to me that as a full time Facebook engineer Koum would have still had time to create a multi-billion dollar startup.

It also seems unlikely to me that he would have been motivated to put in that work for an employer, or that Facebook would have given him the time or freedom necessary to do it.

If he had been hired, he'd have probably been a fine engineer, but there's really no reason to think he'd have added more value than the next guy. The skillset needed to create a successful startup is not the same as the skillset needed to be an effective engineer at a large company.