Remember these charts. They are representative of PV panel characteristic curves.
The box you link looks like a simple battery buck charger. It takes an input, slices it into pulses (quantizes) to cut it down to a lower average voltage. It's taking whatever the panel is giving it and making it match the battery by switching from off to on for various lengths of time. In bright sunlight the panel is outputting say 27V it produces a relatively short duty cycle that averages 14ish volts. In lower light the panel might be at 15V or 20V. The duty cycle would increase to keep the average voltage where the charger wants it to be.
But looking at the panel curves at high voltages it's producing relatively low current. You need current to charge a battery. Not to mention that any time the voltage is above or below the knee on the curve your total power made is lower. So there is a small window where the panel is making its full /power/.
They feed the PV output into a non-inverting buck-boost converter that can produce more output current (or voltage) than the input. This is possible because it's converting power. So rather than just chopping a higher DC voltage down to a lower DC voltage they are attempting to get the full power from the panel to feed into the battery.
MPP is an algorithm that matches a solar cell's maximum power point to the battery. The controller attempts to keep the system operating right on the panel's I-V knee so that the most amount of power is extracted and fed into the charger for whatever the solar energy density (on the left) and module voltage (on the right) happen to fall.
The key here is that it's matching the system, so the panel, load, converters all working to find the MPP. The 'T' part is tracking, which means it uses various control principles to figure out the MPP. The Morningstar uses perturb-and-observe, if you remember your controls theory.
At some times the MPPT and buck converter would be close to equal, small windows of time. But over a whole day, particularly during the middle of the day when the panel output is above MPP voltage, but also for some time before and after as the sun rises/sets, the MPP the buck efficiency will lag a lot behind an MPPT converter. When the terminal voltage is above MPP the buck is throwing away a whole lot of power. The software in MPPT controllers can be sophisticated while a buck converter probably doesn't have a microprocessor at all.