It's painfully simple. When vendors follow an algorithm, they are colluding.

Imagine if multiple vendors in a product or service area form consortium which launches an "independent price determination task force". If everyone follows the recommendations of the task force, they are colluding, even if they don't talk to each other to set prices.

Replacing "task force" by an algorithm changes nothing. The agreement to use the algorithm rather than to compete is the collusion.