As for yesterday’s riddle: not everyone knows what “keivan” means. Keivan is not only “because”, but often “when”. Again, the text is:
מגדף שחזר בו בתוך כדי דיבור אינו כלום אלא כיון שגידף בעדים נסקל
So to translate: A blasphemer who retracts immediately, his retraction is ineffective. Rather, once he has blasphemed before witnesses, he is stoned.
Maimonides is not saying witnesses are the reason he is punished. Once they hear the words, witnesses need not wait any longer to testify.