Exactly. Open source means by design there are more people able to look at the code and therefore more emphasis for those interested in the code to want to make sure it works securely. You can be exploitative and try to keep your hack secret but there’s also a chance that someone else will see the same thing you saw and then patch the code with a PR. Granted it depends on how much the original developer cares about the code to begin with to then accept or write in a patch/fix for the vulnerability that someone else brings up but the example software you listed are larger projects where lots of people have a vested interest in it working securely. For smaller projects or very niche software that have less eyes and interest, open source might not be the most secure.
On the closed source side, the people who are interested in looking for hacks are the ones who are much more motivated to actually exploit vulnerabilities for personal gain. The white hat hackers on the other hand for closed source software are fewer because not having the code available openly means they have to have more motivation (ie the company offering bounties/incentives because they care about security) to actually try to work out how the closed source software works.
I’ll just say that assuming this policy excludes vulnerability/pen testing like Mozilla with Mythos then this is seriously going to be an extremely naive PR (public relations) only move. Obviously actual programmers not bot laziness for coding is essential for software people rely on and I get allowing vibe coding also destroys programmer (and general LLM user’s) brains but to keep up with our now LLM real world reality you need to be realistic not just virtue signalling for purity.