The term "mainline toll plazas" here refers to places wheres tolls are collected if you are continuing on the toll highway (as opposed to places where tolls are collected on entrance or exit ramps).
At the second toll plaza, as you head west from the airport on 528, you don't actually exit the highway, but it looks somewhat like an exit. As you approach the toll location, make sure you're in the right lanes. That will take you to conventional toll booths where you can pay with cash.
Drivers who stay in the left lanes go under an overhead bar with devices that read transponders at freeway speed. Drivers who don't have SunPass/E-Pass transponders and who are paying attention to something else could pass under the bar without realizing that they missed paying a toll. (There are big signs, but people sometimes ignore signs that they think don't apply to them.)
The right lanes and left lanes rejoin right after the toll collection.