I know a lot of these type of articles come from data provided by places like Fidelity who manage 401Ks so they have data but the assumption that people without 401Ks save nothing is sketchy at best.
Some people keep their 401k accounts with previous employers (i.e., the employee doesn’t roll them over when they leave for various reasons). This fact may skew the average 401k balance a bit.
I left a 401K in place because I was comfortable with the plan and the negotiated fees were extremely low. At some point I will roll them over but in the meantime I’m saving on fees that would otherwise be growing along with the balance in the account. The fees really start adding up after ten or so years.