Polish regulators have expressed concerns over possible anti-competitive practices by digital video game platforms. The Office of Competition and Consumer Protection (UOKiK) may initiate proceedings based on the analysis of the collected data.
I think the issue is the same as Apple being anti-competitive about the AppStore. Sure, techically, both Steam and the PSN are platforms for selling games, but they're each restricted to specific non-overlapping platforms (respictively PC and PlayStation) where there they have a (near for Steam, absolute for PSN) monopoly. Hence, anticompetitive behavior.
Steam is not anticompetative at all. On every device steam works on you can download and install a game outside of steam's control with no concern. Hell even on steams actual devices (steam deck) you can install whatever you want.
PSN on the other hand. There is no other source of software outside of the PSN network available digitally for the consoles.