That's the master branch and, as you noticed, it was added after the 26.05 release (that's why I didn't find it when looking through home manager options). So it's likely that after that module is finished and released, niri-flake won't be needed.
(To be pedantic, niri-flake was never "needed", it just provides useful options/functions so you don't have to write them yourself. And it could be the case that it will still provide some things that the home manager module won't, though I can't think of any examples)