He actually wasn't 100% correct. If you get the omnibus "Heavy Planet" edition (including a sequel novel "Starlight" and some short stories), it is explained in an appendix that Hal Clement's estimation of the gravitational field was somewhat off, resulting in erroneous values for the maximum field strength at the poles.

Still, Hal Clement did a bang-up good job, and the error (or correction there of) doesn't negatively affect the story at all.