This article refers to "moderate" (100 lx) light exposure - this amounts to an overhead light on in the room for the entire duration of sleep. I would say an overhead light is a significant amount of light, but wonder how low the threshold is for sleep disruptions. Living in an urban environment, there's a non-negligible amount of ambient light that makes its way in. I don't use a set of blackout curtains, but I know…
Pilots are only allowed to stay in hotels with 100% blackout curtains
It is true that well-behaved airlines, who take an active interest in fatigue prevention, do try to book hotel rooms with blackout curtains and good sound proofing. This is however not a universal behaviour and not all destinations where layover is necessary have any hotels with these qualities.