I think the difference is that you stopped working at 58, and FD is planning to stop working at 53. If FinanceDave had worked 35 years at 53, that would mean that he entered the workforce at 18. If he went to college and didn't work, then he would have had 31 years instead of 35 years. Each additional year he works is making up for one of those zero-balance college years and could make a big difference.
Or a small difference, depending.
The key is "reasonably good income". Suppose FD averaged $65,000 of (indexed) annual earnings for 30 years. Adding in 5 zeros, dividing by 35, then dividing by 12 gives $4,643 of "average indexed monthly earnings". Using the 2010 bendpoints of $761 and $4,586, his monthly SS benefit is $1,950 (or $23,400 annually).
If he replaces those 5 zeros with 5 more years at $65,000, he will raise his AIME by $774. This will provide another $116 of monthly benefit ($1,393 per year). So the number of years he works goes up by 16%, but his benefit goes up by 6%. The small increase results from the fact that his first 30 years of income put him above the second bendpoint, into the 15% bracket. (If FD only averaged $45,000 for the first 30 years, the benefit increase from an additional 5 years would have been twice as big.)
I suppose the extra $1,393 can be viewed as "big" or "small", depending on your perspective. Mine is that higher income people don't gain a lot of SS benefit from additional work.