The difference is that not being "Pythonic" is a style issue. A Python guru may scoff and point out that you could write the same thing more elegantly some other way, but most of the time the performance is the same, and the behavior is correct either way.
On the other hand, not following some of these C++ guidelines can mean your code is subtley broken and depending on undefined behavior, or leaking memory, or accessing something it shouldn't, etc.
I write C++ at work. We all know the language pretty well. We send people to conferences. We have somebody presenting at CppCon next week. We still get bitten by C++ more than you would belive.
You are comparing apples and oranges here. Please compare C++ to other systems languages that are regularly used to write systems / fast / "unsafe" code.
I'm sorry, but I'm not the one who brought up Python.
In any case, even compared to other systems languages, C++ is a mine field. It's sad that C++ and C are so entrenched that you've even equated systems and fast and unsafe, when unsafe really doesn't belong there.
Look at Ada. It performs just as well as C++ and is/was routinely used in low level embedded systems and safety critical systems. It's widely regarded as safe and doesn't have the problems C++ is so often associated with. It was first standardized in 1983 and C++ is just now getting around to adding some of the stuff Ada has had all along.
And then there are the newer, relatively unproven languages like Rust, Go, and D.
I'm pretty good with C, and while C has some safety issues, it doesn't usually bury them under miles of encapsulation. It's also a small, tight language that isn't trying to reinvent its syntax every 5 years.
And there's Rust: it's a pleasure to work with and has almost none of C++'s problems.
On the other hand, not following some of these C++ guidelines can mean your code is subtley broken and depending on undefined behavior, or leaking memory, or accessing something it shouldn't, etc.
I write C++ at work. We all know the language pretty well. We send people to conferences. We have somebody presenting at CppCon next week. We still get bitten by C++ more than you would belive.