We can speculate the pistons are of a high silicon content and that funky crown shape for good combustion may give additional strength.
The Japanese have used steel top rings for years in many applications... being that close to the chamber the 1.2mm ring would have to be steel.
Remember, Toyota sells the TRD supercharger pushing 6 - 7 psi and keeps the 5/60k mile powertrain warranty on this engine. They must have some faith in it. Will it go 300k with one is the real question.
Some info right from Mahle...
Continually redefining limits while improving perfomance
The requirements on pistons for gasoline engines have become ever greater over the past few years. The thermal loads have risen as a result of higher power demands. The stresses at average ignition pressures have increased through knock control, direct fuel injection and turbocharging, while high speed concepts have led to an increase in inertia load. MAHLE has responded to these requirements by utilizing longstanding experience and innovative concepts, which continually adapt the existing product program to the challenges of tomorrow.
The MAHLE program for gasoline engines
The piston program for gasoline engines comprises weight optimized aluminum pistons with diameters from 65 to 110 mm for the highest requirements. The pistons are produced from cast or forged, high-temperature resistant aluminum silicon alloys. These alloys and the refined casting technology from MAHLE enable an ideal combination comprising pistons with low weight and high structural strength to be produced: for example the MAHLE ECOFORM® pistons. Based on the ECOFORM® piston, the next generation of MAHLE lightweight pistons was developed: the EVOTEC® piston. The optimized design enables further weight reduction, while increasing the load-bearing capacity.
New challenges – optimum processes
Modern engines with variable valve train or different direct injection concepts require pistons with a complex geometry. This often leads to a higher piston weight due to involved crown shapes. To achieve as ideal as possible a combination of low weight and high stability despite this, the suitable choice of material is just as crucial as an optimum production process. MAHLE utilizes its longstanding development experience to best effect here, both in forging and casting technology.
Improved coating for less friction
The piston skirt for gasoline engines with cast or steel cylinder surfaces is usually coated with GRAFAL® at MAHLE. GRAFAL® helps to reduce friction and hence increases the scuffing resistance. A further advantage of GRAFAL®-coated pistons: They enable a closer fitting clearance which leads to significantly lower engine noise in conjunction with complex piston forms.