A diff is something that describes the changes between two versions but does NOT refer to any specific version; or at least it is something that can be worked with independent of any specific version.
Ie. I can develop a fix for a issue for version 1.2.30 of some software, generate a the diff using the diff tool and then apply this diff using the patch tool to version 1.1.15 of the software. This might fail (or result in something undesired), but there no principal problem in moving the diff around and applying it somewhere else.
A git commit however is a particular version, so git is not really good at applying a commit somewhere else.
Ie. I can develop a fix for a issue for version 1.2.30 of some software, generate a the diff using the diff tool and then apply this diff using the patch tool to version 1.1.15 of the software. This might fail (or result in something undesired), but there no principal problem in moving the diff around and applying it somewhere else.
A git commit however is a particular version, so git is not really good at applying a commit somewhere else.