I guess the point is that if some people need to put in all effort just to deliver the required thing, while others can deliver the thing and have 20% time left, the latter people have much better time to do documentation.

So you say, why not plan for more time to do the task? Then the people who had 20% capacity for documentation etc. now have 40% capacity for that plus other things, and the same annoyance happens for the other things (e.g. unit testing, automation, etc.).

A good solution requires the company and the team to do the difficult thing and recognize and allow for the fact that different people spend different amounts of time on the same work.

I agree with your sentiment but they were referring to people who do not read documentation provided to them.