No, it's really not, and really hasn't been. Do people truly have such poor reasoning and logic skills?
"Closed source software is inscrutable, impossible for me to fix, impossible for me to review the source" is absolutely a distinct statement from "it is impossible to hide malware in open-source software". I've literally never heard someone claim the latter.
(edit for coherency, thanks graemep)
I think you mean open source in the second bit in quotes.
> "it is impossible to hide malware in open-source software"
No nobody said "exactly that". But many times I've seen people claiming to trust open source as it is safer and people can check and build themselves. Seen it too many times. But reality is different than what is claimed.
It's safer in the same sense as if you're paranoid about your date being a serial killer, you meet them in a public venue. It doesn't mean your date isn't a serial killer, but the risk profile is different because other people can be involved/witness/have context.
You didn't use the word "safe", you used the relative term "safer", and on average, it is harder to hide ill intent in open source software, there's a greater chance it will eventually be discovered. The blast radius is larger for open source (because the barrier to using it is lower), which increases the number of people impacted, but an increase in the number of people impacted also increases the chance of discovery and motivation to address it once discovered.
I genuinely don't understand what you are trying to say.