Yes. You are simply lacking the basic terminology, that’s all 
A “release” in general software talk is “a distribution of the software at a specific state, resulting in a specific version”. Have a look at Software release life cycle - Wikipedia for a longer but good explanation of software release cycles, which includes the notion of a “release”.
So when we say “the 0.18.1 release”, we mean the restic software as it is released/distributed in the state that makes up version 0.18.1 of it".
A release must “be” something tangiable of course, and with software it is most often some kind of release artifact. It can be a binary, an executable that you download, a package with source code, a package that you install, etc.
Those release artifacts are usually made available on a “download” page or when it comes to GitHub on the releases page. So you can go there and download the artifact that suits your operating system and platform, in your case the amd64 Linux one I linked you to earlier.
The “official binaries” means the binaries (release artifacts, i.e. the executables that you download and run) that are the official ones (built/compiled and made available for download by the restic project).
I did not refer to assets, because I was already very deep down into fundamental basic linguistics that normally does not need to be explained. I thought it would be obvious that those are the assets you can download.
Given the fact that pretty much all of the downloadable files there are .bz2 files, and bz2 is a common package format, it’s more or less given that those are the artifacts that one want to download to get that release for the given platform.
The fact that the files have names with suffixes in them are also a common thing. I’m sorry but honestly this is just a matter of you not being very used to this common form of software distribution. You will find this type of distribution in probably hundreds of thousands of software projects, if not (and probably) millions 
At the end of the day, one learn something new every day, all of us do!
I’m glad it worked out 