If someone uses the “cry-it-out” method of sleep training, leaves a baby in a room at night, and doesn’t come back until the morning…
… they are not using the “cry-it-out” method of sleep training.
The cry-it-out method means you sit outside the infant’s room (which is probably your own bedroom, since it’s an infant), and, AS SOON AS THE BABY CRIES, you go in, comfort the baby, settle it down according to your routine, and leave. You do this over and over and over again, until the baby realises that you are not abandoning it, and will always come if they cry.
The process might take an hour a night, for a week. That was certainly my experience. My routine involved a bath, a fresh nappy, singing songs that I’d sung while pregnant, a feed, and watching In The Night Garden, bed, signing “sleep”, more singing, kiss, Night-Night. When he cried, I went in and spoke quietly to him, signing and singing again until he settled, kiss, Night-Night.
He was a very placid baby, though.