Not sure, but I think externally side bolted mains is an expansion control technique as much as a strength thing.
I would guess the caps are ductile iron.
Expansion with alum caps in an aluminum (alloy) block would be too much and the engine would lose oil pressure when hot- iron main caps helps maintain oil pressure.
Ilmor makes their own pistons for the Honda Indy Racing League engines (from solid bar stock!!), but I would guess that these are Mahle forged units...