Software engineering is first and foremost about making good engineering choices. We are proving time and again as an industry that humans CANNOT write mistake-free code.
And when making those engineering choices there are different tradeoffs and constraints to be considered.
The language to use is one of them.
So when a Rust (wild guess) fanboy comes without any background context and makes comments like yours is very telling.
You are correct that bug free software does not exist. But choose a “memory safe” language does not prevent that. A seasoned C++ developer knows how to use memory sanitizers and other tools to guarantee the correctness of its code compared to an average Rust developer that just trust the compiler which, guess what, also may have bugs.
Clickhouse is a Yandex project and at Yandex, they historically use C++ for almost everything, I guess it's part of their culture (probably the founders were C++ programmers?) Their web services such as Yandex Taxi's backend (Uber's equivalent) are also written in C++ which is unusual for webdev nowadays.
You are correct that bug free software does not exist. But choose a “memory safe” language does not prevent that. A seasoned C++ developer knows how to use memory sanitizers and other tools to guarantee the correctness of its code compared to an average Rust developer that just trust the compiler which, guess what, also may have bugs.