Hi - author here - these compound returns are not guaranteed. I reinforce this point under health warnings.

Here the 6% compound returns is an assumption. If that's good or bad to use depends on the purpose of the projection. For long projections in medium-risk funds this might be a reasonable value - but it can also be far off, especially in the short-term.

I made a separate post that surfaces results from an assumed distribution of outcomes rather than a single value, called Visualizing Risk: https://calcwithdec.dev/posts/viz-risk/ (basically a monte carlo simulation)

Also, in my pension calculator example on calculang.dev you can change all the assumptions: https://calculang.dev/examples-viewer?id=pension-calculator (excluding tax relief bits - it's just an example)