Am pretty sure I remember Kizaru blocking with his knees and palms and assuming a pose very distinct from Yasakani no Magatama tbh...
Whatever the case is, the point still stands. If Yasakani no Magatama was, as you said, clashing with Luffy's punches then the simple option for Luffy would be to put more power behind those punches. As long as there is some kind of clash or head to head confrontation, it is sensible to throw in aCoC.
Not using it in order to avoid wasting it would make sense if the problem was that Kizaru was simply outmaneuvering Luffy's attacks and never meeting them upfront but clearly that wasn't what was depicted...