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.