This depends heavily on the employer.
Google, for instance, is extremely friendly towards both side projects (done entirely on your own time and with your own equipment) and open source contributions done either on or off company time.
I wish more employers had the same level of transparency.
Eg they might let you work on some open source project, but if you wanted to commercialize some side project, it's an entirely different can of worms. (And different clauses I'm your employment contract.)
The article alludes to that, albeit from the company's point of view. The same principle applies to you.
If you ever want to sell your side project in the future, the buyer will put you through the wringer called "due diligence." If you were employed at XYZ Co. at the time you developed the IP being sold, part of the due-diligence process may require you to prove that XYZ Co. doesn't own the rights to it. If you kept it secret from XYZ, you won't have a signed waiver from them, so you won't be able to do that. Instead, you'll have to go back to XYZ Co., hat in hand, and ask them for a waiver now.
That will be an awkward conversation.
Also, this formalizes standing on both sides; both parties will know more clearly where each stands.
Of course you may proceed without sharing, but might also end up losing more later. Calculated risk.