The fact it's open source enabled someone outside the project to find it in practice. While also possible with closed source software, if you think the bar is possibly too high with an open source project, it is an order of magnitude higher with closed source.

Also, please don't say "Google". A bunch of hackers (on Google's payroll) found it, not Google. We can't tell what would've happened in a counterfactual universe where Google was not financing Project Zero.