Thanks - so what problem were you seeing with this?
This is the clinical trial they are reporting on:
In South Africa, where the B.1.351 lineage is prevalent and 800 participants were enrolled, nine cases of COVID-19 were observed, all in the placebo group, indicating vaccine efficacy of 100% (95% CI, [53.5, 100.0]).
If no-one is infected in your intervention group, that is indeed a efficacy of 100% - so don't see an issue?
I see the rest of the page describes trials in additional population, including their large Phase 3 study, which resulted in slightly lower efficacy.