Maybe most C++ applications are low-value as attack targets, so no-one has bothered to find all the corner cases that make them blow up.
The fact that applications like browsers and operating systems (which are known to be high value targets) have a lot of effort & resources put into security but still have attack vectors makes the "C++ is secure" position fairly indefensible.
pcwalton mainly works on web browser development (Servo), which whilst sharing some goals with game development, also differs in some respects. Although online security is more and more important in games these days, the real appeal of Rust in respect to game development is in providing an alternative to the 'death by a thousand cuts' that can plague large C++ projects.
I've posted a list of the things I consider the most relevant to game development: https://news.ycombinator.com/item?id=7587413 Any one or two of them alone wouldn't really be a compelling enough reason to switch, but put together they form a very compelling value proposition.
The fact that applications like browsers and operating systems (which are known to be high value targets) have a lot of effort & resources put into security but still have attack vectors makes the "C++ is secure" position fairly indefensible.