Really? It makes absolutely zero sense to me. How can Elf years be different from human years if it has the same number of minutes, days and months? Dev. just saying Elves are slow mentally, because it got to justify living for 300 years and having learned nothing. If that's the case, would they take 100+ years to reach maturity, learn to walk, speak, etc.? But hey, I've read much, much worse lore in Japanese VN's so oh well.
I'm almost certain every fantasy elf (not santa clause) lives for hundreds, or even thousands of years yet develop much much slower. For example, Legolas is 2931 years old but he looks like he's in his mid - late 20s, maybe early 30s. This is very much how I've done things, just with different scaling.
Common Elves haven't learned nothing in 300 years. Before humans arrived, they were fighting with bows and spears. Within 300 years, (equivilence of 20 human years) they went from spears and bows to guns, skyscrapers and sentient robots.
Common Elves would take 270 years to reach the development of an 18 year old human. This is why Elves have so few children since they have to take care of them for so long. They're a fragile species.
This really isn't anything new, I think almost every elf in fantasy lore is exactly like this, but with different scaling.