It's not about it being more secure. It's that at least when it's suffering from malfunction, you got shitloads of eyeballs on the thing to make it work and work well. Why do you think all the OpenBSD tools are so ubiquitous?
In some ways, it's best not to think too hard about this.
If it were open source you, or anyone who was fitted with the pacemaker could look at the rules that control it.
Making _anything_ visible and open to criticism instantly makes its creators more accountable and adds pressure for them to try and be responsible.