Different people can have different versions of the word “collusion” but the important definition is that in the law, and in most countries it does not require an explicit agreement between parties.
I’m not aware of any definition of the word collusion, legal or otherwise, that doesn’t require an agreement between parties. I can’t see how you can have collusion without colluding.
Tacit collusion still requires a tacit agreement. From your linked article:
>In an article published in the Journal of European Competition Law & Practice, I argue that such an approach would amount to a reversal of the burden of proof under existing case law, in particular on the prohibition of coordinated facilitating practices. Indeed, following the current position of the Court, in order to establish the existence of a collusive agreement solely on the basis of circumstantial evidence (such as the uniform adoption of an alleged facilitating practice), and without proof of reciprocal contact through direct or indirect communication, the existence of any plausible and legitimate alternative explanation must be ruled out.