Beyond parallel decoding >>124 of the deflate stream using the seek points, which produces runs of filtered image rows, parallel unfiltering can also be used. The condition is that the first row after a seek point may not use the previous row, so it can only use None or Sub filtering, not Up, Average or Paeth.
6. Filter Algorithms
https://tools.ietf.org/html/rfc2083
The encoder that produced the four images of >>121 >>122 seems to take a much simpler approach and uses Sub filtering for every row.
AJlNWhC.png
1604x1238 8-bit RGBA(6)
filters: [0, 1238, 0, 0, 0]
hLzyLXE.png
1598x1238 8-bit RGBA(6)
filters: [0, 1238, 0, 0, 0]
iQSyNAQ.png
1612x1246 8-bit RGBA(6)
filters: [0, 1246, 0, 0, 0]
Vc2eT5I.png
512x562 8-bit RGBA(6)
filters: [0, 562, 0, 0, 0]