In practice, the only thing that really matters here is the phrase "his work was falling behind". And "he was missing deadlines". It doesn't matter if a worker

In practice, the only thing that really matters here is the phrase "his work was falling behind". And "he was missing deadlines". It doesn't matter if a worker is secretly pulling 80 hours or 10 hours a week instead of the 40 (apart from legal/contract issues). If he's doing less than agreed between the parties, that's a problem. If management bought his excuses, then there's a problem with management.