Google has a similar clause in their employee contracts. I assume most tech companies do.

That doesn't mean it is enforceable, though.

In contrast to Google, Microsoft, my former employer, probably has (had?) the best policy amongst big tech: moonlighting wasn't just tolerated, but actively encouraged! (...provided it runs on Windows, of course) ...because it's basically free training/experience if it means exposure to new APIs/platforms/libs/concepts - and definitely helps the morale levels of folks who love to build things but who ended-up with an extremely narrow-scoped job at the company (e.g. PMs who don't get to write code, or SDETs and SREs that only get noticed by management when they don't do their jobs).

During the launch of Windows 8, Msft's moonlighting policy was also part of their Windows App Store strategy: we were all heavily encouraged to make an "Windows Store App-app" so that SteveB could claim MS had N-many apps in its app-store, because that's how Leadership thought they could build credibility vs. Apple's established app store (of course, what actually ended-up happening was hundreds of cr-apps that were just WebView-wrappers over live websites).

In contrast, I understand Apple might have the worst moonlighting policy: I'm told that unless you directly work on WebKit or Darwin then you have to deactivate your GitHub account or else find yourself swiftly dragged onto the proverbial Trash.