The lack of Memory Safety in C and also in C++ is clearly a major security and a major safety risk.
For obvious reasons, Mr Stroustrup is painting C++ in the best possible light. This is a bit unfortunate, as great men will acknowledge the limits of their inventions. Of course many things are a trade-off and C++ errs on the side of performance while not being too much focused on safety and security.
The latter two aspects are better served by Ada, Rust and Sappeur.
Real-world programs contain programming errors and a memor safe language can contain the adverse effects of these errors in lots of cases.
Regarding "reliable systems": there exist large, complex programs which are extremely reliable:
+Jäger 90 flight control
+Airbus A310 and later flight control
+many ABS brake systems
There exist processes such as V-Model, ASPICE and ISO26262 that enable a team of engineers and their leaders to create such reliable, complex programs. It boils down to careful documentation, careful coding and careful testing. Documentation and coding is done on several levels and in a very systematic way.
Unreliable software/electronic systems are due to quick+dirty engineering, a lack of proper funding, lack of proper leadership and inexperienced engineers. A lack of "engineering honesty" is also a major cause of dramatic faults.
Finally, "forcing a system into production" like the V-22 OSPREY is a major source of accidents. They killed more than 40 soldiers because of that, while the competing Doriner 31 project did not kill anyone. Also, the Do 31 delivered better performance in speed and payload !