You don't actually have to directly communicate with someone in order to collude, you just have to both be knowingly working towards that same end (tacit collusion).

https://www.winston.com/en/blogs-and-podcasts/competition-co...

But that's aside from the ridiculousness of suggesting that BUs are so independent that their actions aren't being viewed in total by the shared management they both report to.

ELT at Amazon is responsible for the outcomes of their BUs, negative ones included, whether the individual BU leaders 'knew' what those outcomes would be or not. In fact, that's literally how it's supposed to work; ELT directs strategic outcomes from the top.