Only in certain states, and I only if you sign such a contract. Did the author actually read his contract and say: "Please remove this part"?
I know California and Washington both have state laws limiting what companies can do in terms of taking ownership of your "outside" work.
My current employer technically forbids me from showing photographs of my dog to my mom. Naturally, no one would ever care if I did, but my contract explicitly considers any distribution of anything I produce that can be copyrighted (at work or not) an act corporate espionage.
In large corporations, an exception to a blanket IP contract would require approval by fairly high ranking people in both HR and Legal. When you have >10,000 employees, individual exceptions simply don't happen for anyone under any circumstances.
My experience is only reflective of very large companies. I doubt it's representative of smaller employers.
My last employer was a startup and I had this stricken... as well had privisions in place so I could do outside consulting as long as it wasn't in a competitive industry (this they may have given me as I came to them initially doing part-time consulting).
The place I worked at before, a 15k person corp, is a place I wouldn't have even thought of asking for things like that. It would have a couple weeks just to have gotten a turnaround through HR and legal.
https://www.exratione.com/2011/11/the-miserable-state-of-int...
Most contracts I've come across don't define working hours versus not.