Excuse my noobness, but from this official windows binaries build page, if I understand correctly, you need to download a fresh clone from github.
BUT, my intention is to build from the file I downloaded, which is Electrum-3.3.8.tar.gz which I extracted to Electrum-3.3.8 directory. I only want to build Electrum Standalone Executable from this directory and not from a clone from github.
Is this the only possibility? Can anyone confirm? Yes / No
If cloning from github is the only possibility to do this, then I've set up an unrealistic goal from the start which only became apparent now

You don't need to clone it from github again.
You can definitely use your downloaded (and extracted)
.tar.gz.
Just skip the following commands:
$ git checkout $REV
You can keep the rest as it is.
Just make sure that your extracted archive is in the folder "electrum" as it would be when cloning the repository directly and you are fine to go.