How is the way it dates itself different from the way that a period story dates itself?
A period piece (or any fiction set in the past) explores a certain time period socially, culturally, politically or however the writer desires, in the course of its story. Regardless of when you read it, you're learning something substantial about the time in which the book was set.
Ready Player One is a dystopian near future story whose running theme is nostalgia over the 80s. It's not actually set in the 80s, nor does it explore the 80s in any meaningful way beyond the most superficial pop culture aspects of it. If you, as a reader, have no personal connection to the 80s and 90s, Ready Player One becomes an average adventure story about a guy playing a video game. Unless you're specifically interested in that nostalgia element for that exact time period relative to the 2000s and 2010s, there's really nothing to set Ready Player One apart from Log Horizon.
At least Log Horizon is a story about MMO culture, which can be of interest to young adults in the future. Ready Player One doesn't even have that. Its understanding of MMO design and gameplay is... tenuous at best.
Although I guess it could be unintentionally hilarious in the coming years: