Maybe this could be rephrased to reflect the fact that there are other tools which aid with pinning, but that flakes is ~upstream (unlike something like niv).
This tool is designed to work only with flakes. It’s not clear to me that mentioning Niv or other approaches would be particularly edifying here.
Right, but is it true in the absolute that dependency pinning is flakes’ killer feature? Not clear to me, considering stuff like Niv exists and does this too.
The killer feature (for me at least) is that it does that while being (or having a chance of becoming) part of standard Nix, with all that this entails (see e.g. the discussion here).
Meaning you don’t need to mention Niv, just scale down (or qualify) the claim a bit.
I think that “quite possibly” is sufficient qualification for the last paragraph of an announcement blog post. If this were a highly technical deep dive I’d be more inclined to agree.