Lee Sedol was not the best player anymore at that time (not saying it wasn't an impressive/important achievement, but overstating it doesn't help either - the "beat best human players part" came later in 2017).
Lee Sedol was still top 5, certainly no worse than top 10 at the time. By all mean he wasn't the best and most dominant, but the difference with the top was tiny.
I don't understand who's downvoting you, this is accurate. While AlphaGo/Zero improved quickly to superhuman play, we are just in this thread comparing timelines, so that is relevant.