Sites like these use different methods to detect what browser you're using.
Mega.nz does the same thing.
It has very little to do with security and more to do with supported features.
The sites are hard-coded to detect user agents (there are other methods as well but that's out of the scope of this post)
Edge provides this user-agent.
The site probably isn't updated to understand edges user agent so it serves up that page, to get you to download and use a browser that it understand. This is so they don't have to spend money on support.
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2311.135 Safari/537.36 Edge/12.10240
If you''re wondering why Microsoft's browser has a user agent with things like "Mozilla" "AppleWebKit" "Chrome" and "Safari" I'd suggest you read this: History of the browser user-agent string