Maybe it shouldn't cause backpressure if done properly, but maybe some construction companies carelessly block one side of the pipe while it's curing?
EDIT: Nevermind, I originally was thinking that a sufficiently negative pressure differential could overwhelm the capacity of the vent. But the more I think about it, the pressure range that I'd expect even with an unusual operation probably wouldn't be far enough outside normal range to do that. ...although there are definitely still some possibilities to do some stupid.
On the top floor of a building I worked at long ago, there was a drain in the floor of the men's bathroom. It had a p-trap but on windy days there was enough vacuum from the vent stack to drain it out. I encountered a neighbor pouring a cup of water into it one day and he explained the problem. I started doing the same, and had to do it every couple weeks during windy periods.
But I think that's a problem for the upper floors, both due to fluid dynamics and because the first trap that loses seal would mostly restore equilibrium.
Anyway, everything uses PVC, right?
This would only be temporary, but if gases have accumulated near the trap they could diffuse back through the trap and into the room. These would seem to be exceedingly temporary situations however and not likely a source of chronic exposure to pipe gas. The only exception here might be if some HVAC imbalance keeps the ambient pressure in the room at 3-5" of head pressure over the vent.
They never once implied the fumes were entering via the plumbing.
In fact I will quote: "Emissions from a nearby CIPP job got indoors through cracks in the building’s foundation and irritated workers to the point that they evacuated"