It's not a failure if the design spec was exceeded, that's how those sorts of things work. If a defensive system is rated up to X, and you exceed X, the defensive system has not failed, it has simply had an attacked that was beyond what it was designed to defend against used against it.
Iron Dome previously had a 90% intercept rate across fifty days and 4K rockets fired, with ~70 rockets actually managing to land that were fired from gaza.
That was over fifty days, the muzzies managed to launch five thousand rockets over the space of just one in this attack. The defence system probably still maintained its 80-90% intercept rate, but it was just a simple case of the sheer volume of munitions fired meant that more made it through than when the attacks were spaced out.
I think the real failure, was A) The muzzies managing to punch their way through the Israeli border seemingly without effort, and B) The Israeli military intelligence missing this attack coming. (If they did indeed, miss it coming.)