but that implies that the shipping company was offering overtime as a retention or perk tool, which is far fetched.

If they cut overtime by more than 30-40%, or the point the googling tells us the longshoremen would come up short, then that seems retaliatory in nature more than good business sense. It means the shipping company is either too bloated in operational expenses (the unjustifiable overtime expense to keep longshoremen happy with lower hourly wage) or it means the shipping company is making a decision to generate less revenue out of spite for being cajoled into paying a sharp wage increase. Both of these options would indicate the company is not operating according to free market principles. There's really no third logical option here that you're arguing.

The company is likely operating according to market principles. Overtime will exist, even after the company agrees to a higher wage.