Look at it this way...

What type of air is being expelled out of that mechanical vent? Before you answer that question, let's assume that the vent does NOT work (burned out).

Now, what is being expelled out of that vent--Gravity fed warm/ hot attic air (whether working or not). What is being sucked in thru the plumbing vent-- nothing. The vent is only a positive/negative pressure relief.

Now, I'll ask... what is being drawn into the home? Everything from both the non-working (worst case assumption) mechanical vent and the plumbing vent are gravity fed-- nothing is being sucked in.

Is everything in the code perfectly written... nope-- but most are. This just happens to be one of those that you ask yourself, does it cause a safety concern? Does it have the potential to cost the homeowner a monetary issue? Does it create a health issue? Does it create a functionality issue for either the plumbing or attic ventilation?

I just think this is one of the "small fish" that won't make me loose any sleep at night over.

Rich