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The upstream repo seems to use bazel as its build system, at least
according to the README. Is that not usable here? The bazel tool
appears to be packaged in bazel-bootstrap in Debian.
bazel-bootstrap is very old, unfortunately --- it's bazel 4.2.3, while the current version is 6.something and has moved on a lot since then. bazel's really hard to package.
The sad thing is that bazel, which is the least bad build system I've ever used, works in a way that's completely antithetical to how Debian wants to build things: it doesn't want to use host software for anything and will, e.g., download and recompile libraries you refer to at build time. So you end up with mostly static binaries containing embedded copies of any libraries you refer to. It even likes to download its own compiler rather than using the host one. This is all for perfectly good reasons, but those reasons don't work for Debian.
It ought to be possible to write bazel files which do produce Debian-compliant binaries, but it'll be tough (you'd basically need to replicate the ability to install Debian packages into the bazel build tree in order to use them as dependencies), and of course any third party software won't be doing that anyway so the package maintainer would need to rewrite large chunks of the build system.