TL;DR No one knows how to build them. More info through The story of Windows releases since 2.38 - #6 by scnorth and Compiling dot dlls for 64 bit on windows (#1218) · Issues · graphviz / graphviz · GitLab.
I currently don’t understand this. It should be “Win32” also for development binaries. This is set by the $Env:platform environment variable in .gitlab-ci.yml. PowerShell however has the peculiarity that environment variable set in a child process also affects the parent process since $Env: is a drive which means that all bets are off 
. This needs to be further investigated.
EDIT: The reason is that Set up VCTools variables and import into PowerShell environment changes this environment variable to x86 (Search for platform in this log). This setup was introduced after the latest stable release in Set up VCTools also for MSBuild tests (9afa76f2) · Commits · graphviz / graphviz · GitLab, hence the difference. I’ve written an issue about it.
We introduced them since the latest stable release in Drive all tests from pytest (!1483) · Merge requests · graphviz / graphviz · GitLab.
We only host the latest stable release because we don’t have disk space for all of them. The 2.38 release was a manual mistake by me and I’ve now removed it. It and other old Windows releases are instead found at Index of /Archive/stable/windows.