This fix definitely works. I had the same issue, but I’m not sure if it is a FCP bug. All the progressive quicktime files I rendered in AE (tried all codecs: Animation, ProRes, etc) show up in VIDEOSPEC as UNDEFINED in the INTERLACING field and look bad jagged in a FCP progressive timeline.
Then I inspected other progressive footage made in other apps and they clearly show up as PROGRESSIVE in the INTERLACING field. These files show up fine in FCP in a progressive timeline.
I don’t want to point fingers but there might be something weird going on with AFTEREFFECTS rendering program. Even footage de-interlaced in Adobe media encoder shows up clearly as PROGRESSIVE in VIDEOSPEC, then why is AE rendered footage showing up as UNDEFINED in the Interlacing field?