I don’t know if Free software makes software less secure but I don’t see how it makes it more secure? Especially given that security might not be a high priority for the authors.
I’m not a programmer so it’s hard for me to understand the arguments, but from what I gather is that any popular software could be a vector for threats and that users themselves remain the biggest vector.
The author suggests greater use of code signing could help. It also seems to me that progress towards computer security needs to be the province of large, well resourced organizations.
Our own (US) government seems to not be prioritizing this correctly, preferring to maintain their own exploits at the cost of defense. This is a political problem. It is of a piece with the historical arrogance of the US security-military apparatus that is being badly outplayed, mistaking pure military superiority as the most important type of security.