So the initial $600K did practically all the work. The additional $800 per month is just a thin layer of icing on the cake.

Yes, without the additional monthly contributions, the $600K would have become about $1.99M anyway, falling just a bit short of the desired $2M. The additional $96K of monthly DCA'ing resulted in adding another $200K to Joe's nest egg. So the end amount of $2.2M put him comfortably over the top of his goal.

BTW, the monthly DCAs had a much bigger effect when Joe was younger. You might ask - how did Joe end up with $600K at age 48 in the first place?

Again, you can work backwards and figure out what Joe needed to have started with at age 38 by using the DCA calculator. It turns out that in 2004, at age 38, Joe only needed to have had $200K in his index ETF. By contributing the $800/mo from his paycheck the next 10 years towards purchases of VOO, his 401K would have grown to $600K by 2014. If he had NOT made those contributions, he would only have had $434K in 2014, and would NOT have been able to retire early this year. The monthly contributions added almost 50% more to his nest egg.

S&P 500 DCA Calculator: ofdollarsanddata.com - Dca calculator

Remember that during 2004-2014, there was a nasty, vicious Great Recession of a bear market during 2008-09, and another nasty near bear market in 2011. Joe still wound up tripling his next egg with the DCA method during that period. And then, during the second 10-year period, from 2014-2024, Joe lived through two more nasty bear markets - the Covid one in 2020, and the more recent bear in 2022 - and still wound up tripling his best egg again and retired early.

Mechinv
For success in investing, your emotional fortitude counts for way more than your intellect. Your objective is not to "look smart" and impress other people that way. Your objective is to be financially independent, so that your time is totally your own.