Consider the Faraday Cage or shield, a structure through which signals are to some extent prevented from "squeezing through". Understand that they are not perfect, and the degree to which they isolate the interior basically depends on the design and expense of creating the shield, and the wavelengths of the transmissions they are intended to block.
Next consider the construction of ordinary buildings in which no specific efforts are made to shield the interior. Certainly, metallic elements of such structures do to some extent attenuate signals entering or leaving the structure, they are never entirely blocked.
If signals can't "squeeze through", then your cell phone would never work inside a building at all.
It's not correct to say signals only enter or leave through "openings" however, they do penetrate non-metallic parts of structures. Glass, drywall, wood, and other materials are all transparent to electromagnetic waves to some extent.
I suspect the "engineer" you are disparaging simplified the explanation to focus on the observable effect rather than the mechanism and the mathematics behind it.
Yes, my brother explained it that way because he has a big screen TV and we thought it would be great to show his kids and mine "Back to the Future."
The kids asked many questions after they saw the movie and one of them was how you can still talk on a cell phone when you are in a building and there is a lot of cement all around.
He was not speaking to his fellow engineers, he was speaking to kids.
My metaphor, about the truth Mike Lindell was presenting and it would slip out no matter the barrier, was lost a few idiots.