Language purity for a project is never bad, it makes it easier to contribute to a project. However, this is barely a feature, given that C is weakly typed and common C functions do not use boundary checks.
For an operating system that aims to be secure, one would expect a more progressive stance, and heavy (intellectual) investment in a language that makes safety easier.
Yes, I know that OpenBSD prefers more secure variants of common functions.
When you only have 1 program using another language and it slows the builds, you tend to want to replace it. There are still scripts and such in different languages (ksh and I do believe some perl).
Since OpenBSD still compiles on a VAX, removing C++ from the build is a decent feature. As you say, OpenBSD has function with bounds checking and that seems to satisfy their requirements. Also, they haven't achieved language purity and I don't think they are trying.
For an operating system that aims to be secure, one would expect a more progressive stance, and heavy (intellectual) investment in a language that makes safety easier.
Yes, I know that OpenBSD prefers more secure variants of common functions.