There are Glock parts that are almost certainly MIM - the firing pin, the firing pin safety plunger, and the locking block immediately come to mind. But like every part in a Glock, they appear to be designed as part of a system that is very tolerant of widely varying component dimensions.
MIM is not bad technology - it's successfully used for many automotive powertrain components (stuff like connecting rods, transmission gears, ring and spider gears in diffs, etc.). Sig's problem isn't the MIM process, it's the excessively fussy design and the dogshit component supply chain. Like most polymer pistols, the cost-of-goods-sold is so ridiculously low that there is zero excuse for outsourcing critical firing control group components to India. Go talk to a domestic company that makes critical auto/medical/military MIM parts, pay 2-3x for parts that actually meet the print, reap the financial benefits of not discharging live rounds into your user base.