It’s the crux of the law suit? They are claiming that valve are applying it to non-steam key games. I think this is their website steamyouoweus.co.uk/faqs/

These price parity clauses apply to all games listed on Steam, not only those distributed via Steam Keys. As a result, other platforms cannot offer better deals, limiting consumer choice and keeping prices higher across the board. This harms competition in the market and stops other platforms from improving their services.

Though I do think the last part is nonsense.

It also says it in the article, though I suppose it is less clear:

The lawsuit - filed at the Competition Appeal Tribunal in London - alleges Valve “forces” game publishers to sign up to conditions which prevents them from selling their titles earlier or for less on rival platforms.