Why are PC and console split into subcategories? Are there major differences that set them apart from each other or is it just that loading times are different? I want to run this co-op with someone and they have the console version and I have the PC version.

Yes, loading times, gameplay and it's local co-op that would require the same platform, I don't believe PC can play with a console player even locally.

What's different about the gameplay? I thought PC and console versions would have had the same gameplay.

As far as I can tell after one play through, there are no differences other than loads. I don't think that would be enough to warrant subcategories for PC and console.