Yes, Reddit's source code is published on GitHub. But that doesn't mean that this is the exact same code that is powering the website. There is absolutely nothing stopping them from applying any modifications they want on top of the published code, or even running an entirely different version of it.
Open source software is great, but only if you can verify that you are using software built/running from verified source code, and there is no way to verify that with a website running on someone else's servers. When a web service states that the source code of their service is published doesn't mean that you can undeniably trust it just based on that.