Double Miracle Time Weekend

That doesn't prove anything because they literally said the exact same thing as Nexon did. That's more of a translation issue than confirmation.

I guess we will have to just wait and see

But they didn't literally say the exact same thing Nexon of America said. The Korean event notice doesn't only say that you can rank up twice, it specifically says that you can go from Rare to Unique, in those exact words. In the text with the 02 in the big yellow circle, it says, in purple text, 큐브사용 시 레어(Rare) -> 유니크(Unique)까지 한번에 두 단게가 상승합니다.

This thread on Insoya also says you can't go from epic to legendary, but the response is based on the event notice, so take from that what you will.

It is not, however, confirmation that you cannot go from Epic to Legendary here, just that you couldn't in Double Miracle Times in Korea.