I'm not sure if I follow. How does "not trying to hire each others employees" lead to wage cartel? Wouldn't they instead have to agree to fix their wages for that to happen?
Apple pays employee x, Google offers x*1.2, Apple could retain by offering x*1.2 (or more). Apple instead decides to collude with Google to not poach from each other. Now all companies everywhere decide to do this. Wages get fixed.
It's a negative pressure on wages if you manage to get hired at one of the top companies in the country, but are unable to jump to one of the other large companies. It effectively removes the most stable companies from that employees prospects, only allowing growth if that employee moves to a smaller company.
Just to be sure, did they completely refuse to hire each other's employee, or did they just agree to not send emails poaching them?
Empirically, within a few months after publication of Facebook refusal of a "truce" with Google, Google implemented a 10% raise across the board, and a $1,000 bonus.
This was part of the lawsuit. They communicated with each other about offers made to each other's employees and agreed not to counter-offer above the initial offer.