From my experience, being on both sides of this equation, the things missing from sole-developers who want to move to a company are a few important ingredients - writing clean, easy to read code, architecting in a scalable manner, writing good quality tests, working in an Agile environment (using real tools like Jira - with regular standups and sync ups) and being very good at git (both in creating pulls and receiving/performing code reviews).

Code reviews are the biggest learning experience you can get, and you don't get that at all as a sole developer.

Otherwise, most of these things can be learned as a sole developer, but you must seek out the knowledge and apply it to your projects. If you worked at a company from the start these things would have been ingrained in you.

As far as working in an agile environment, you don't necessarily need this experience as most sole developers who than move to an Agile environment will actually just become better and more efficient once they are introduced to the company's structure.