[thwarted]

It's safer in the same sense as if you're paranoid about your date being a serial killer, you meet them in a public venue. It doesn't mean your date isn't a serial killer, but the risk profile is different because other people can be involved/witness/have context.

You didn't use the word "safe", you used the relative term "safer", and on average, it is harder to hide ill intent in open source software, there's a greater chance it will eventually be discovered. The blast radius is larger for open source (because the barrier to using it is lower), which increases the number of people impacted, but an increase in the number of people impacted also increases the chance of discovery and motivation to address it once discovered.