Agree partly, but partly not.
My understanding is :
While in closed loop control, the goal of the control system is to achieve perfect stochiometric mixture.
However, there are 2 factors that make it a bit more complex than simply keeping the mixture correct all the time.
1) Our (old fashioned) narrow band O2 sensors give about half scale output when the mixture is stochiometric, but the sensors are very sensitive. They have a very steep gain curve, and only indicate the mixture accurately over a narrow range above and below stochiometric. That's why they call them "narrow band".
A little lean - it pegs at negative full scale = 0.0V or so.
A little rich - it pegs at positive full scale = 0.7V or so.
2) Three way catalytic converters work by temporarily storing chemicals so it can make the opposite chemicals combine, because it's dealing with both unburned hydrocarbons and excessively burned oxides of nitrogen. Tough job. So it needs rich - lean - rich - lean.
So - that's what the control system does on the upstream side (O2 sensor #1)- rich - lean - rich - lean.
But it's still shooting for stochiometric mixture, but only on average.
The catalytic converter does the averaging, and the downstream side (O2 sensor #2) should verify stochiometric, a smoothed waveform - about half scale.
Control system engineers call this sort of thing a "bang-bang" controller.
It's one of the least sophisticated types, but actually works pretty well in the real world.
Old fashioned furnaces and air conditioners work this way. Most car A/Cs do.
So, like
@Heckraiser said.