The conclusion that patients with depression sleep too long is often based on measuring time in bed rather than actual sleep duration.

“when you examine what people were asking those patients where there is this conclusion that patients with depression can sleep too long really what they were asking in those studies was what time you go to bed and what time do you wake up and there what you clearly find is that people with depression will be in bed for significantly longer periods of time and the inference there and you could argue almost the conflation is that if you're in bed for longer then you're sleeping for longer and therefore depression is a condition of hypersomnia”