I don't know why you're so unwilling to believe those screenshots are from Trump's social network. Dozens of different people independently posted screenshots of the network, including some trusted reporters. It would be quite a conspiracy to suppose all of them coordinated and lied about those images. Such little to gain. Such massive consequences.
If you need more evidence, here [0] is the archived login page from the mobile version of the site before it was taken down (because it was flooded and trolled). By comparing it visually to other mastodon instances and inspecting the source itself, you can confirm there was a mobile mastodon instance up at the given archival time. Additionally, the press release [1] links to the social media website [2] which links to the app store page [3] which has preview images that are very similar to the images posted on Twitter.
There were dozens of people who created accounts and were generally trolling the new platform. They didn't lockdown the mobile site (https://mobile.tmediatech.io/) which let people register accounts and post to the platform. The links / screenshots are definitely to his site.
