Imagine you wrote a WYSIWYG text editor, like Libre Office Writer. You have all sorts of functionality and an overall architecture which makes it sane to upkeep the project & have things work well together. Then someone else makes a custom font, but kind of does it their own way and with a different approach making it a one off from the way the rest of the fonts all work and are used in the program maybe using a custom font file format parser and different UI element even though you know it could have just used the normal, already maintained and planned out code paths.
You can of course merge anything with the right license if you so like, like that one off font code into your editor, but if it doesn't fit well into the overall project or meet the general quality standards of it then it's not practical to and can actually be worse than not including it. Upstreaming is about submitting something the maintainer can reasonably accept and maintain, not just about whether working code is available. GPL licensed code provides the latter, it's still up to someone (either the original company or some other interested person) to make it fit right first.
You can of course merge anything with the right license if you so like, like that one off font code into your editor, but if it doesn't fit well into the overall project or meet the general quality standards of it then it's not practical to and can actually be worse than not including it. Upstreaming is about submitting something the maintainer can reasonably accept and maintain, not just about whether working code is available. GPL licensed code provides the latter, it's still up to someone (either the original company or some other interested person) to make it fit right first.