Absolutely is. It also has steel re-enforcement's added into the frame for the slide to ride on.
Nylon 6-6 with a 33% fibreglass filament re-enforcement, to be precise.
It has a higher strength to weight ratio than 6061-T6 Aluminum (not as high as 7075-T6) and is much more flexible and less likely to develop stress fractures.
(Strength is based on Yield value of tensile strength)
Nylon-66 with 33% Fibreglass: 150-ish MPA at 1.14gm/cm3
6061-T6 Aluminum: 270-ish MPA at 2.7gm/cm3
7075-T6 Aluminum: 503-ish MPA at 2.8gm/cm3
(just a quick look at these values online, so keep that in mind)
It's also a thermal insulator, impervious to corrosion and in the case of a catastrophic failure; it will crack and release pressure before it turns into a hand grenade.
Bit of an HK guy, they are great handguns, so if you like it buy away! Although unless you're getting a good deal, I might look at the newer HK45/P30 as they're much more ergonomic in the hands!
ETA
Well I saw the OP change it to the USC45, so fuggit time to add another picture!
The same can be said for the USC45, polymer receiver with steel inserts on high-wear areas; IE the Trunnion block and the rear where the bolts hold it to the lower, as well all the steel inserts for the picatinny rails.