It's so painfully obvious; he never needed a buff in the first place and it's 100% out of the blue. Also it's on his W slow amount (which is probably the thing least affected by buffs), an obscure value that can give Kha some boosted playrates and buyrates.
Nobody buy this skin. Please protest this shit
Maple Nectar commented on this via Twitter when a player asked a similar question: https://twitter.com/RiotMapleNectar/status/1044664008949690368
It's primarily a pro play focused buff because we felt that Kha'Zix is on the verge of competitive viability with small room to grow in solo queue (he was sitting at 48-49% WR according to our internal data before the buffs). W evolve Kha'Zix (prior to the stint Kha'Zix had with R evolve first earlier this season) has seen play historically and was likely the way to go for a buff to avoid negatively impacting players.
When my gal Sona gettin them E-sports buffs yo She's been 0% pickrate for what....3+ years now
/sp
