That's complete bullshit. I've known plenty of plumbers, I've never met one who had almost $1 million saved up by 33--which is the example they are using.
Sure some small percentage of plumbers might make $90k after 5 years and manage to seriously save. But that's not anywhere close to normal.
Also even with the cherry picked example of a union plumber who is unbelievably disciplined with respect to saving. And a doctor who does an optional fellowship.
The doctor still comes out WAY ahead over their lifetime.
And it's generally not a life threatening career.