This is the correct answer. You don’t HAVE to open source it, but if you even email a copy of the code to one other person (outside your organization if your company owns the copyright) then the GPL applies.
> if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
https://www.gnu.org/licenses/gpl-3.0.en.html
If you don't want to open source your shit, don't distribute it to others.
Discussion
If you change code on a GPLv3 project you have to open source it under the GPLv3 period.
Wrong. Only if you distribute the code at all. If you keep it on your own hardware (or on your company’s hardware) and never give it to anyone, you don’t have to open source it.
If that's true, have you ever wondered why AGPL exists? The only change is that it considers the use of the software over the network as distribution (such as an API or web interface).