You’re the one that said that when studies adjust for these differences they no longer show a correlation.
That is just flat wrong. You used that exact phrase “adjust for”. You can move the goalpost if you want to “I don’t think they are correctly adjusting for all differences” but that’s a very different argument than the one you presented.
The preponderance of the evidence is that fiber has a protective effect against colon cancer and there are multiple plausible explanatory mechanisms.
“I don't know what notion people have about common Western food practices decades ago, but in the 1980s, for instance, fibre was basically non-existent in the common diet”
This is misleading.
1. There is a single gram per thousand calorie difference in aggregate between 1980 and today, so if it was non existent then, it’s non-existent now.
2. Percentage of ultra processed food and food away from home has gone up drastically since 1980, so the types of fiber consumed on average have changed drastically as well.
3. “Fiber” is now a much broader category than it used to be. Resistant starches can now be labeled fiber. If a tiny fraction of the population eats the equivalent a single low carb tortilla with 30g of resistant starch per day that’s enough to completely throw off the aggregate fiber stats. There is much less evidence that resistant starch is protective against colon cancer.
4. Stability in the aggregate doesn’t preclude changes in sub populations. Changes in the way we eat leading to a larger sub population with very little fiber intake.
We know that there have been changes in sub populations. For instance one study that found fiber consumption was stable overall between 1999-2017 found that it dropped among White adults, but the aggregate was stable because it rose among Black and Hispanic adults.