It absolutely does. Each utility is a program all by itself and the compatibility tests apply to each and every one of them. How fast they are is not relevant until they are 100% drop in replacements.
It might not apply for the whole project, but it certainly does apply for each individual per-command functionality (cp, mv, or even the general cross-command features like the filesystem interface).
[0] https://github.com/uutils/coreutils/tree/main/src/uu