MS Edge seems to think the Windows Scrivener Beta is malware. Had to jump through some hoops to download it.
You probably have your SmartScreen settings turned up. SmartScreen is a Windows technology that relies on a combination of developer digital signatures (developer pay for a certificate which they then use to sign all binaries they release) and other algorithms and heuristics to determine the likelihood that a new download is actually malicious. When it’s turned up at its highest, it will warn you about anything that it can’t absolutely guarantee is a valid download and force you to jump through hoops (as you have found) to actually use them – which is useful, because it can indeed help block the automated download and execution of genuine malware.
At any rate, it’s not at all unexpected that beta software would trigger a false warning. I don’t know if L&L is doing digital signatures on the beta downloads, and if not, that alone would be enough to generate a false warning if your SmartScreen settings are at their most paranoid. You can safely ignore it, and perhaps may want to dial down SmartScreen if it bothers you. I, personally, like having my settings up and having to take a few extra seconds to check things out. Better that than having to recover my files and rebuild my OS.