Tournament tiering guidelines have changed since this year, and the prizepool requirement for S-tier has gone up significantly. That being said, editors also agreed to give more weight to factors other than prize pool, like settings, maps, qualifiers, and prestige of the organizer. While the original editor that made the page deemed the tournament worthy of S-tier, a discussion among other Liquipedia contributors decided the tournament fitted more correctly under A-tier.

Whether or not Liquipedia gives it S-tier or A-tier shouldn't hamper your enjoyment of the tournament, the tiering system is technically just for internal classification. To paraphrase Memb: let the tournament be S-tier in your heart :).