It is much easier to find security vulnerabilities if you have the source code.

Better switch the Linux servers to Windows then

The source code for Windows is available if you meet certain requirements: https://www.microsoft.com/en-us/sharedsource/

Only applies if there are significant bugs. Presumably Linux has fewer of them than Windows, but it's hard to say as Windows gets a lot of attention.

Then maybe Apple leaked this code intentionally to harvest free security reviews?

Please, it doesn’t help anything to publish unfounded conspiracy theories. Apple already publishes open source projects and has a bug bounty in which they pay for security vulnerabilities in some areas of iOS. It’s ludicrous to advance the theory that they hatched a plan to leak this important project to Reddit instead of through one of their official channels.

Risky strategy, it depends on who does the review. (It should be a motivator for white-hats to dig into this - black-hats surely will).