https://support.delta.chat/t/help-testing-the-upcoming-v1-58-x-release/3737
From the link you provided! Its the only download available on that page as far as I can see. I found the instructions quite confusing actually. ...~test 1.57.0 for release 1.58.0 with a 1.59.0 core...
"Please checkout & test the new Desktop Prerelease 1.57.0 before we publish the final release 1.58.0"
