The flan seems corroded and it's bright due to a probable aggressive cleaning, with removal of the patina. This can sometimes be mistaken with cast process. But of course, it's not impossible to be an ancient cast forgery, indeed.
This one I purchased for $25 a couple of years ago. The detail is not good and it has a serious flan crack: HADRIAN AE Sestertius OBVERSE: HADRIANVS AVGVSTVS P P, laureate bust right, draped far shoulder REVERSE: HILARITAS P R S-C, COS III in ex, Hilaritas standing facing holding palm & cornucopia, two small children flanking her Struck at Rome, 128-134AD 25.30g, 35mm RIC 970