LFortran author here. As kergonath said below, both "new" Flang that you linked above, and LFortran started at about the same time. I know the Flang developers well and I am hoping we'll be able to collaborate in the future more. It looks like MLIR would be one option: they are going to use it for Flang and LFortran can have a backend that could also use it.

LFortran's main advantage is that it is interactive, so the parser and semantics has to be a little different. And also we designed it so that it will be quite approachable for people to write tools on top.