> security through obscurity

Security is security, how you get there is entirely up to your organization. Also I'm not sure how having code access to their site proves that it isn't some rich person's tax avoidance scheme. In a way, all charities allow rich people to avoid paying taxes to some degree.

why is “somebody wants to make money” a bad thing?

It's a bad thing when the job is advertised: "Volunteer/Unpaid".

The poster I replied to indicated that the “making money” part could come later.

It is easy enough to have a volunteering agreement that catches this with clauses. Open sourcing the code won’t help with that scenario anyway.

But if it were open source, volunteers could fork and self-host the application and create their own competing non-profit/organization.

It’s a content-driven website with user submissions. They don’t even claim copyright or ownership on the content, which is the actual thing that makes up the unique value.

Open source what, exactly? You point makes literally zero sense