I've wondered the same thing. I assume it means time in bed, otherwise the recommendation would need a disclaimer about sleep efficiency and time in bed vs actual sleep. Some people are able to get by with around 6 hours of sleep, I've assumed these people have higher sleep efficiency while the 8 hour recommendation is for people with around 85% efficiency.
If it meant “time in bed”, then they’d need the disclaimer about “efficiency”. Someone who takes an hour to fall asleep would be “inefficient” and need more time in bed, but they wouldn’t actually need more sleep.
