If it's this thread you mean:
https://www.photrio.com/forum/threa...sources-on-e-6-process-and-slide-film.195404/
The only one who said something about your question on grain made a comparison between old slide film and modern C41 film. It doesn't really go in the direction of the summary you have above.
This is not to nitpick, but your summary just didn't match my own experience at all. Back in the 1990s, our family switched from shooting slides to color negative because ...well, long story, but we wanted prints. Immediately, everything went grainy as heck. We used to project slides a couple of ft. across without very apparent grain and now we were looking at 5x7" prints that were decidedly grainy.
So maybe my memory is somehow tainted. Although when scanning the family archives a few years ago, I had the same experience revisiting those same images. And when I got back into film at any meaningful scale around 2003-ish, I again had the same experience comparing e.g. Sensia 100 and Sensia 200 to e.g. NPS160.
I don't think I ever attempted to track down the datasheets of the films that form the basis of the memories/experiences above in an attempt to compare RMS granularity specs. But the pattern for me was quite clear. Was/am I wrong? Maybe...?
By means of illustration - not that it's a scientific proof or anything, but still:
Sensia 200
View attachment 354226
Fuji NPS160
View attachment 354227
Same scanner, both 3200dpi 100% crops, no sharpening or noise reduction applied, no in-scanner grain reduction, smoothing or ICE.
I dunno about that.
Sensia 1600, pushed to 3200:
View attachment 354228
Fuji 400 color negative (rebranded for a local retail store):
View attachment 354231
Sensia 400:
View attachment 354232
All examples exposed in 2004, on freshly purchased film, lab-developed and scanned on the same scanner.