In addition, one advantage of still splitting Header and Footer, even with a Layout, is you can make them optional props to App with what you have as the defaults so they can be overriden.
It could still be the same file, but the typical convention is one exported component per file where the filename matches the component name. This is just a style guide for recommended best practices and readability.
I was answering this question. No, no issue. A HoC is just an enhancement to reuse the layout. See my self reply with more info on file splitting by single export convention. It is just for ease of navigating a repo and human-readability. Is it needed for a small project? No. My personal preference is to be consistent so I usually follow the same conventions regardless of project size.