The service, the website it’s running on and the phone app were all new things created for Trump though. I don’t see anything saying anything about what the back end runs on.
Let me use a huge internet company that isn’t tied to Trump and isn’t a conservative thing for a comparison. Netflix uses at least 3 different open source Apache products as part of their tech stack, and that isn’t the **only **open source stuff they use. No one is going to argue that Netflix is lying about providing a new/separate service just because there’s quite a bit of open source in their stack, especially on the back end (including stream processing).
Hell, there’s a term LAMP that has been used because the specific combination of Linux OS, Apache web server, mySQL database and PHP scripting was so fucking common - all of those are open source and at least one of those is part of the back end of a lot web sites you likely visit (including Wikipedia, Facebook and Slack which all use LAMP). Apache is the web server software for something like 30% of websites.
It’s the same thing with Truth Social and Gab - they built a new site, running a new service , with their own newly coded mobile app, that runs modified Mastodon as a core part of their back end. The AGPL (the license Mastodon is under) also requires distributing the source they use, including any modifications, so the source code behind both sites is freely available.