Your link says they are comparable, with another study claiming better quality for open source software. While I really don’t think that grouping together vastly different softwares based on this one quality is meaningful (is a proprietary CRUD app made in 5 months with an ad-hoc architecture comparable to an open source database with proper planning and domain-expert programmers working on it?), I also didn’t claim that they were worse off, just that more eyes won’t find more/all bugs, because in fact, they don’t actually look for them in the first place.

The fact remains, every type of software is ripe for bugs and security measures must be taken so those bugs won’t become exploitable.