What is not substantiated?

Practically everything the EU DMA/DSA addresses was highlighted in the "House Antitrust Report on Big Tech" back in 2020.

https://www.nytimes.com/interactive/2020/10/06/technology/ho...

But if that's not enough, since then:

Apple's policies banning developers from referring customers to alternative payments has been widely ruled illegal around the world, first and foremost in the USA where they were even referred for criminal investigation for continuing to do it after being court ordered to stop.

Google has been twice convicted of antitrust monopoly abuse in the last year in the USA, and found to have exploited user privacy settings several times.

Meta's harmful practices have been continuously revealed in court: allowing sex trafficking and prostitution to help train their AI, allowing scam ads because they're profitable, deliberately exploiting children spending in games because it's profitable, and illegally tracked users.

Amazon's antitrust for exploiting vendor data is ongoing, so I guess you can have a point there.