Kibi Do you think downloading from github you compile from the code or something? You download binaries that you have to trust were built from the same source code you are seeing. Obtainium is for cases where you trust the developer but don't trust the storefront e.g. Play. If you are afraid of a malicious dev there is absolutely nothing protecting you from one when using Obtainium.
Accrescent is simply an app store that follows the modern security standards. It does metadata verification so there is no need to do it yourself. It is not made by the GOS devs AFAIK though its developer has an account here.