It was said in the Naoki's corner #1,
https://www.twitch.tv/videos/399663821 Time stamp ~4:59:00
They talk about job changes and they talk about Paladin and oath stance may be getting some changes, then the woman mentions WHM, they joke about Yoshida sitting between two white mage players and he Hmms and Haaas a bit after saying he's a black mage and then he says 更に使いやすくなるんじゃないかな I guess they'll become even easier to play/use. だっと思います (or so I think/At least I think so). The girl is actually happy with this response. 白は最近ね石しか投げないよね He then jokes about how all WHMs can do now is throw rocks -_-;
He is definitely not joking around when he says the part about making the white mage easier to use. But, yes, he's really not revealing anything by saying that. But the tone of his voice is serious, so it is a serious answer. It's not a sure answer maybe he's not fully sure of the details himself, as he does say "or so I think" afterwards. But this is basically an ask me anything discussion about jobs, so he is being serious before the jokes about the rock throwing.
Sorry about the edits. I've been playing it back and forth a few times to make sure I'm hearing what he's saying correctly and trying to get more of the context but that's all there really is.
It could mean just about anything and it really depends on who is the one "using" the whm? Is it going to be easier to use it in a raid? Easier for the player to control? We assume the latter and I'm sure that's the answer that's implied, but what that involves, it's anyone's guess really. It could be wonderful, it could be a hot pile of garbage it could just be... "meh." We won't know till May and that's a long time to let anxiety build up again to explosive levels should the news not be good.