In many ways, distributed open source software gives more social attack surfaces, because the system itself is designed to be distributed where a lot of people each handle a different responsibility. Almost every open source license includes an explicit disclaimer of a warranty, with some language that says something like this:
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
Well, bring together enough dependencies, and you'll see that certain widely distributed software packages depend on the trust of dozens, if not hundreds, of independent maintainers.
This particular xz vulnerability seems to have affected systemd and sshd, using what was a socially engineered attack on a weak point in the entire dependency chain. And this particular type of social engineering (maintainer burnout, looking for a volunteer to take over) seems to fit more directly into open source culture than closed source/corporate development culture.
In the closed source world, there might be fewer places to probe for a weak link (socially or technically), which makes certain types of attacks more difficult. In other words, it might truly be the case that closed source software is less vulnerable to certain types of attacks, even if detection/audit/mitigation of those types of attacks is harder for closed source.
It's a tradeoff, not a free lunch. I still generally trust open source stuff more, but let's not pretend it's literally better in every way.