I was thinking that most people would say that if my goal is X, and I achieve far less of X than I easily could have, then that would qualify as a ‘mistake’ in normal language
I see where you’re coming from but I actually agree with Michael.
In reality a lot of people are interested in contributing to the common good but actually aren’t interested in doing this to the greatest extent possible. A lot of people are quite happy to engage in satisficing behaviour whereby they do some amount of good that gives them a certain amount of satisfaction, but then forget about doing further good. In fact this will be the case for many in the EA community, except the satisficing level is likely to be much higher than average.
So, whilst it’s possible this is over pedantic, I think “the claim” could use a rethink. It’s too late in the evening for me to be able to advise on anything better though...