The only claim anyone has ever documented is detailed in the article.

An accusation doesn't necessarily mean they're right though. Something people get confused on often is Steam Keys, which are completely separate to Steam Store purchases. Valve do ask developers not to "give Steam customers a worse deal than Steam Key purchasers"

You can read through all the claimants key documents if you like 😉

https://steamyouoweus.co.uk/updates/

So far no one has ever shown Valve asking for price parity with other outlets, and this doesn't appear to be any different. Just a lawyer looking for a payday.