I don't see how a company could have prevented an employee from leaving voluntary.
The article states that the company agreed not to use the most aggressive recruitment techniques (cold calling) on each other, which sounds like a gentlemen agreement more than collusion.
Google states that they recruited from Adobe, Intel, Apple, etc., that's pretty easy to verify.
They agreed not to actively compete for each other's employees.
In my simplistic (possibly wrong) view;
If you look at employment as a market place, and individual employees as different commodities, it keeps the market price of employees (specifically the extremely talented ones) artificially low. In the extreme it would be similar to price fixing. This is good for Larry and Sergei and Steve, but not good for the rest of us.
They shouldn't be making any agreements about hiring practices. (Gentlemen's or not).
I think your view is spot on. This was large companies engaging in collusion to artificially hold down wages in the labor market.
The article mentions that employees should know their worth, but doesn't mention how? The only way to know is to either actively seek a new job and get offers or be approached through cold calling. This ruling clearly helps the employee.
He said the practice would repress wages. Absolutely nothing about leaving voluntarily. It seems pretty reasonable that an agreement banning the most aggressive forms of recruitment would have some negative effect on employee compensation.
The article states that the company agreed not to use the most aggressive recruitment techniques (cold calling) on each other, which sounds like a gentlemen agreement more than collusion.
Google states that they recruited from Adobe, Intel, Apple, etc., that's pretty easy to verify.