FOSS isn't adequate protection against leaks.
1) Code is TL;DR and draws upon libraries that draw upon libraries, so assume nobody read it.
2) Developers can make mistakes or simply not foresee an attack vector.
3) Even the stuff that is audited is not necessarily audited in every version.
4) Gov devs and malicious hackers spending months or years building trust is a thing.
5) Nostr adds the honeypot of controlling zaps, to the hacker motivation. Someone could eventually become very wealthy by simply collecting nsecs quietly and then suddenly zapping themselves everyone's wallet contents and disappearing.