Legally there is no such thing as "collusion" without people in a room talking to each other
Not so! The Apple eBooks case from a decade ago is a great example of this.
Apple's agreement with the publishers had two important conditions:
1. An agency model that allowed publishers to set their own prices while Apple took a percentage cut.
2. A Most Favored Nation (MFN) clause that ensured Apple's store would always have the cheapest price for any given book.
The combination of those two factors ensured that prices would go up industry-wide because Amazon, who wasn't part of this, was still operating on a wholesale model. Trying to match/beat Amazon's price in Apple's store—a requirement of the MFN clause—wasn't tenable, so the only alternative publishers had was to raise their prices with Amazon to match the prices they had at Apple's store.
Even though none of the publishers sat in a room together or made agreements with each other to raise prices across the industry, they were all aware that the others were entering this same agreement with Apple and understood the effect these conditions would have on the industry. They knew that their collective actions were going to result in prices going up, hence why they all settled out of court as soon as regulators came after them for colluding. Only Apple tried to fight it out, and they lost.
I'm reminded of a cooperate card game called Hanabi in which players can't see their own cards. They are instead only permitted to provide limited hints about the cards others are holding. Those other players then read between the lines to understand the logical implications of the hints that have been given. In much the same way, algorithms of this sort can be recognized by others and, once discovered, amount to an implicit, open invitation to engage in price fixing. Whether or not that's "collusion", I actually don't have a stance (I may argue that you can collude without ever meeting each other, but that doesn't mean Inecessarily think that's what happened here), but I think it's fair to say that this sort of algorithm should be illegal, due to its very nature.