The message is that you can have the greatest investment vehicle in the world right in front of you, but without understanding your own human frailties and applying consistency and discipline to your investing, the chances are slim that you will be able to best take advantage of it. I don't care if you are a die hard indexer or have all your net work in bitcoin, this holds true for all types of investors.
I'm not sure I follow the logic of this claim when it comes to someone who puts everything in an index fund. Surely the reason why most lose money in managed funds is because they mistakenly believe they can time the market. This is true for managed funds as much as attempts by individuals on their own to pick stocks. The die hard indexer pursues a simple strategy characterized by humility (during the accumulation phase) of consistently making regular contributions to an index fund, ignoring the daily noise of the stock market. The index investor wins out in the long run not merely because of low fees, but also because he or she is not dancing in and out of the market. Maybe this is your point, but if so, I don't know what it would mean to be an index investor if you are suggesting he or she might nevertheless foolishly undermine their strategy by attempting to time the market.
I would absolutely not assume that indexers are less likely to try and time the market compared to those whose funds are managed.
Tons and tons of antsy index investors here post constantly either about market timing or needing to be talked off the ledge of market timing. Meanwhile, people who have to go through their financial advisor can't do anything rash on impulse because they have a line of defense in the way between them and cashing everything out because they read a scary news story about Trump.
I work for a finance firm, the FAs I know spend A LOT of time and energy preventing people from being reactive.
There are also the wonderfully clueless people who dump their money into mutual funds with an advisor because they know nothing about investing, I think they're actually the most likely to just leave it alone because they literally don't even know that market timing is an option.
Perhaps you could posit that of the self directed investors, the indexers are less likely to market time than the individual stock pickers, but I don't know that that's true since the value investors tend to have a buy-and-hold approach as well.
All in all, I actually suspect that it's the very novice indexers who read just enough to start indexing, but not enough to feel confident, who check their investment numbers 100 times a day, and are constantly freaked out by even small drops; I suspect they're the ones who jump in and out the most.
The benefit of indexing is the lower fee drag, not that the person is less likely to market time.