It is a downside but you can avoid it by not looking at the EE code you plan to contribute. Looking at the EE code is optional, it won't be needed to contribute to CE. The GitLab company will solve any merge conflicts that arise.
I responded to Sephr's comment. We think the open source project is better because of our open core business model. More than 80% of the company development efforts go into open source code. Many people contribute new features, but the work to make installation easy, release management, fixing bugs and performance regressions, investigating security reports and updating dependencies is for the most part done by GitLab the company. I recognize that the open core model requires a careful balance. But I think it is working very well for GitLab and is one of the reasons GitLab became more popular than the 100% open source alternatives.
