It was announced by Yoshiki, and shown today on his birthday livestream.
Yoshiki is one of the most famous musicians in Japan. The main relation to BABYMETAL is the immense influence his main band, X Japan, was to Koba. BABYMETAL has done dozens of homages to X Japan, here is a list of a few.
Thanks to Kapanak for the recording and funnytoss from the BABYMETAL Discord for translating.
Quick rough translation:
Su & Moa: Hi, we are Babymetal!
Su: Yoshiki-san, Happy Birthday! In 2014, we met at a performance in London, and though it was the first time we met, you told us that our performance was interesting and cool, and those encouraging words stuck with us and give us inspiration even now. And then after that, you invited us to X Japan’s show at Yokohama Arena, and also praised and talked about BABYMETAL in magazine interviews, which made us very happy
Moa: From the bottom of our hearts, we hope that Yoshiki-san had a wonderful year!
Su & Moa: Happy birthday, Yoshiki-san!
Yoshiki then said some comments regarding BABYMETAL:
Yoshiki kind of rambled, but basically the gist of what he was saying is that he was inspired by Babymetal as a Japanese, and felt he had to up his own game as well when he saw them in the UK