MPPT controllers are not required for series panel wiring. Counterexamples:

1. Imagine a 24v battery bank being PWM charged by two 12v panels in series, for example. The series configuration is not important, but the relationship between panel voltage and charging voltage is.
2. Imagine a the 24v panel string above MPPT charging a 12v battery bank. The MPPT controller dies and a backup PWM controller is installed to keep power flowing. PWM will run the panel string at Vbatt, making roughly 1/2 the peak power the MPPT could make. If it were three 12v panels in series then PWM would make 1/3rd the peak power. Note that during float with no additional loads there would be no practical difference.

“MPPT required for series” is shorthand for “when Vmp greatly exceeds battery bank voltages then PWM is hamstrung and MPPT advantage is most compelling.”

The corollary to this is “when Vmp is close to battery voltage then MPPT advantage is thinnest and PWM may actually make more power.”