There is this working alternative:
Unofficial UE4 Launcher for Linux (download assets from the marketplace) (GitHub - nmrugg/UE4Launcher: The Unofficial Cross-Platform UE4 Launcher)
I used a google linked account as a signin method (just in case you try something else, this method is working!)
This other is not currently working:
I have already posted this answer in unreal engine forum, but my posts are awaiting approval and I read that may take more than a month! so, as there are many linux users struggling to download even things they bought, and we dont want to have to install windows for nothing, I post here in a hope they reach here in some way, when they google again about it!