Wat? How would you translate an ISO hash into anything meaningful to compare to the actual source code (that you don't have access to, but that's beside the point) ?
Does microsoft enforce reproducible builds[0]? Even if they did, you couldn't reproduce it (no access to source). When you hash a large binary, like an ISO, you cannot deconstruct what, eh, sub-binaries it is made of, and you cannot magically decompile those, eh, sub-binaries into source code.
Backdoors are subtle bugs introduced into the existing codebase. They are compiled into the build.
They aren't added to the binary after the compilation by a malicious third party.
In Ken Thompson's "Reflections On Trusting Trust" Turing award lecture from 1984, he talked about "trojan horses" being inserted into the compiler in such a way that any code run through the compiler results in a program that has behaviour which is unable to be determined from examining the source code.
They are "compiled into the build" by a malicious _compiler_, not from malicious bugs introduced into the source code.
It's quite obvious that such behaviours could easily be things which deserve the name "backdoor".
(It's only short, and _well_ worth the time needed to read it if you haven't come across it before: https://www.archive.ece.cmu.edu/~ganger/712.fall02/papers/p7... )