Not sure what you mean by this. Most retirement accounts are investment portfolios. I don't think they meant that people should reinvest their entire portfolio in the offering, or invest money they couldn't afford to lose.
I think he means that stock picking is way more dangerous than a well diversified "safe" Fund like most retirement accounts push you to invest in (with high management fees of course).
Retirement accounts can just as easily invest in Vanguard funds, side-stepping the 'high management fees' issue of target-date funds. Picking is more dangerous when you're playing to retire vs. playing to build wealth. Further, your asset mix should become more and more conservative over time.
The typical guidance is to shift towards bonds and short-term instruments in a fund or ETF over time. The problem with that is that the yields have been poor for many years.
A nominally safe investment like the Vanguard Short Term Gov Bond ETF is down in real and nominal terms over the last decade.