It was using RGBA instead of a palette before.
$ zopflipng -m place-indexed-final-place{,_opt}.png
Optimizing place-indexed-final-place.png
Input size: 3175277 (3100K)
Result size: 976309 (953K). Percentage of original: 30.747%
Result is smaller
Rendered output is bit-identical:
$ convert place-indexed-final-place.png ppm:- | sha1sum
fccae2655eaea6e3950bf337824d239298162f51 -
$ convert place-indexed-final-place_opt.png ppm:- | sha1sum
fccae2655eaea6e3950bf337824d239298162f51 -