Without any insider info, there are several things odd with this:
1. Harmonious is not known for having a holiday tag and there is nothing Christmasy about it.
2. The things have to work. Remember the issues with Rivers of Light.
3. It would be really odd to have fireworks in EPCOT but not MK, and there's been no news about that.
4. A lot of what happens with Harmonious happens atop the lake, which would create crowding along the shore to see it. Disney would have to do what Universal did and rope off viewing sections all the way around the lake and have distanced circles on the ground and only let in one party per circle... if WDW is still caring about such things in a state that doesn't care. And that would be a major undertaking in crowd control.