they mean time in earth moves 10 times slower than time in the angel world (ie 2 years in angel world = 20 years in earth)
wait put the wrong label it the other way around 20 years in angel world is 2 in earth
I know what the author was thinking, but it's not what was said. Quote, "I lived with angels and in their world time moves 10 times slower". That sentence is saying the angel's world is the one moving slower, not the human world. What it should have said, "I lived with angels, and in their world time moves 10 times faster. While I was up there, 15 years passed in the mortal world. Which means I lived 10 times that."
The author wrote the original in Korean so it probably was a mistranslation or a certain phrases/word didn’t work well in english but understand the confusion from that line luckily they talk about it more later on which explains it better
It's not 10 times slower, it's 10 times faster. If it were ten times slower he'd still be a brat, because less time would have passed for him. Seriously, he only would have aged by like one and half years. Also, previously he was six, so why was he now eight?