IIRC, while they couldn't cheat the need for squarings, each squaring/multiplication itself has some internal parallelism which can be exploited, and GMP does s

IIRC, while they couldn't cheat the need for squarings, each squaring/multiplication itself has some internal parallelism which can be exploited, and GMP does some of that, and the Cryptophage team took that even further in conjunction with their exotic hardware. So the serial step could be accelerated more than Rivest predicted.