I agree that your citation sounds strange; at least, it does to my ear. Reading that I almost wonder whether the author hasn’t somehow conflated progeny and prodigy, since the latter’s plural form is unremarkable.
However, digging deeper, one finds that the OED entry for progeny, last updated in 2007, makes no mention one way or the other. It gives as its sense 1a
1a. Offspring, issue, children; descendants. Occas.: a child, a descendant; a family.
a definition that is already in the plural, but which admits a singular sense “occasionally”. Sense 1b is more figurative:
1b. fig. Spiritual, intellectual, or artistic descendants; successors; followers, disciples.
And is still plural, and gives this citation of progeny itself taking plural concordance:
- 1994 H. Bloom Western Canon ii. iii. 80
Dante’s progeny among the writers are his true canonizers.
However, down under sense 1d:
1d. The product of the breeding of animals or plants; the offspring of sexual or asexual reproduction. Now chiefly Agric. and Genetics.
We find that there is indeed an inflected plural version — progenies — given as an example:
- 1977 Crop Sci. 17 909/2
Twenty-one clones whose polycross progenies ranked high for rate of seedling emergence under field conditions or had high forage yield..were selected for this study.
So it appears that when dealing with people, progeny is taken as a plural with some use as a singular, but when used in a genetic sense, it can occasionally be taken as a count noun that inflects regularly.