Newness is at best only partly true. As far as total packages go, Alpine definitely loses to all three in your cohort. No need to guess, plenty of statistics at https://repology.org/repositories/statistics

2 points in relation to those numbers. First is that distro's don't package things the same way. Debian is most famous example for how they break up packages into many smaller ones compared to the other distros. Ie. comparing raw numbers doesn't work exactly. Secondly we are talking about packages that are used in Docker images, a definite subset of those for general purpose distros. I've always found Alpine to beat the other distros packages on these terms.

Indeed it’s not perfect. But relying on one anecdote is not better, either. Further, it’s difficult to justify the claim that Alpine has more packages than nixpkgs when the raw numbers are >7x the other way.

Anecdotes are also known as experience and this wasn't part of a scientific paper, it was an expression of my personal experiences using these tools. My most recent test of this was working on a container based setup for writing software and tracking the tools I needed across fedora, debian, nix and alpine. Alpine was the only one that had full coverage and kept up with versions appropriate for the use case. Other cases (more hosting oriented) were longer ago and may not be valid these days, but had similar results at the time (though not nix, it wasn't a contender back then).

It has to do that because upstream software won't have Alpine-specific build pipelines. (Though I expect a lot more do nowadays.) Meanwhile everyone tests on Fedora, Debian, and Ubuntu so if it isn't in the distribution repository, or the version isn't up-to-date, it's as simple as having your Dockerfile fetch and build from upstream.