Security breach isn't confirmed, though FortNite did have a vulnerability to play as other accounts, I don't believe it involved the store or using login credentials.
Also you can believe him or not but Sweeney denies the claims the store was ever compromised:
Epic Founder Tim Sweeney Denies Epic Games Store Security Breaches
Innocent until proven guilty?
Lastly, security breaches are serious matters and I'd always be skeptical of the business however I'll go ahead and assume you're still using Facebook, Steam, banking institutions, among many others who've had breaches. For your convenience here's a list of companies you can boycott :
List of data breaches - Wikipedia
(Epic isn't listed, Steam is)