The US 5 door ECU is completely Speed Density based. That means MAP sensor based and both the TIP and MAP sensors are used. The MAF sensor is simply used as a temperature sensor in the intake. The 3 door non US Fiesta STs have a completely different ECU operating system and they DO use the MAF sensor for airflow readings.
The car calculates the mass airflow you can datalog using the MAP sensors. It uses a complex HDFX system that Ford has patented for determining the airflow entering the engine. The MAP pressure signals, engine speed, camshaft positions, volumes of manifold and intercooler, turbocharger/engine modelling, temperatures, barometric readings are all used to determine that final airflow number which is then used to determine fuelling, load, timing.