The WHO report is very clear: red meat and processed meat cause an increase in risk of some cancer. And they're very clear that it's causal, not just correlation. And they're clear that the risk is increased by 15-20%. But that increase is on a very low risk.
"This is a perfect example of confusing correlation with causation."
You are confusing two different fields of science. This isn't physics where an action causes a reaction. This is biology and the human body is a massive ecosystem.
The 800 studies that "eventually" reached the WHO is the best way to determine a connection between two things. This is decades of research and maybe around 1million human subjects that were studied in total.