You don't have to have an absolute monopoly. Microsoft was punished for anti-competitive actions in the 90s when it bundled its Internet Explorer browser into Windows.
Even though other browsers existed, like Netscape Navigator etc, Microsoft was still found guilty of violating the Sherman Anti-Trust Act and guilty of anti-competitive action, both in the US and elsewhere.
Microsoft was forced to create a version of Windows without IE bundled, and other corrective actions.
To this day while I welcome anti-trust legislation, I don't understand how a "free" thing can be anti-consumer. Both Microsoft and Netscape were not "charging" the customer for this product. While it could be argued the product may generated revenue, the purpose of anti-trust in general is to stop something that negatively affects consumers.
Now the food industry would be my best example that needs more anti-trust. All of my organic brands have been bought out by Conagra Brands. There's no competition.
I would also agree that this specific acquisition is not going to be helpful for consumers.