Because these are ranked by prevalence x severity, and most programs are written in memory-safe languages, the UB-related weaknesses are probably at the top of the list for programs written in C or C++, especially because "ordinary" servers are usually not written in those languages. But the point is that, indeed, even within these unsafe languages, not all kinds of unsafety are equal.

It is true to say that memory-safety issues are among the biggest problems in C and C++, but it is not true to say that unless you absolutely prevent them all you remain in the same spot. If you significantly reduce or prevent some of the memory safety issues, you're in a place that's not too different from that of programs in memory-safe languages.