Aha, wasn't that argument more about that closed source software is more likely to hide stuff you don't agree with, than FOSS? Not necessarily that FOSS won't have any viruses or malware, but it's at least less likely. That was my take away, but long time ago I read the book admittedly, I might misremember or transformed it automagically over time.
This is my takeaway as well. Having the source code open makes it auditable, if not by you, maybe the community.
The free software license specifically gives the software an extra advantage in that changes to the software must be shared openly, if distributed as as binaries.