You don’t need a lot of skilled wallet users, just a few is enough to make the presence of any bugs available to community.
Even Bitcoin Core has flaws and vulnerabilities which are identified, or even on occasion not identified before they were exploited, despite significantly more pairs of eyes on its code than on the code of an individual wallet. Despite how technically competent an individual reviewer is, more reviewers will always be safer. And you will get more reviewers if your code is open source and those reviewers have an incentive to spend their time examining your code.
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I don't disagree with any of that, but their code is still not open source and to call it such is simply incorrect.
Is reproducible does not mean it is open source or the site is not correct about it.
Read my previous replies in this thread. The source code is reproducible but it is not open source.