Hey! Author here :)
So let me start by saying that that blog post was written was 15 years ago and I don't even remember the details of it and what I've written there. But, I have a hot-take on this topic you've touched on!
From a programmer perspective, you are absolutely right. The behaviour is undefined, end of discussion. A programmer should never rely on what they observe as the effective behaviour of an UB. A programmer must avoid creating situations in code that could result in the execution flow venturing into the areas of UB. And - per C and C++ standards - results of UB can be anything (insert the old joke about UB formatting one's disk being a formally correct behaviour).
However, I'm a security researcher, and from the security point of view - especially on the offensive side - we need to know and understand the effective behaviours of UBs. This is because basically all "low-level" vulnerabilities in C/C++ are formally effects of UBs. As such, for the security crowd, it still makes sense to investigate, understand, and discuss the actual observed effects of UBs, especially why a compiler does this, what are the real-world actual variants of generated code (if any) for a given UB for this and other compilers, how can this be abused and exploited, and so on.
My point being - there are two sides to this coin.
So let me start by saying that that blog post was written was 15 years ago and I don't even remember the details of it and what I've written there. But, I have a hot-take on this topic you've touched on!
From a programmer perspective, you are absolutely right. The behaviour is undefined, end of discussion. A programmer should never rely on what they observe as the effective behaviour of an UB. A programmer must avoid creating situations in code that could result in the execution flow venturing into the areas of UB. And - per C and C++ standards - results of UB can be anything (insert the old joke about UB formatting one's disk being a formally correct behaviour).
However, I'm a security researcher, and from the security point of view - especially on the offensive side - we need to know and understand the effective behaviours of UBs. This is because basically all "low-level" vulnerabilities in C/C++ are formally effects of UBs. As such, for the security crowd, it still makes sense to investigate, understand, and discuss the actual observed effects of UBs, especially why a compiler does this, what are the real-world actual variants of generated code (if any) for a given UB for this and other compilers, how can this be abused and exploited, and so on.
My point being - there are two sides to this coin.