The counter-battery radar doesn't prevent artillery from working; it makes it dangerous for them. Theoretically the units that took this out could already be destroyed after having had their coordinates calculated and counter-battery fire immediately called down on them.
In practice it was just setting up, having been tracked to its location, and possibly wasn't working yet. Also the GMLRS rockets fired by HIMARS are not ballistic - they execute a counter-battery-confounding turn. And the salvo is fired quickly after which the vehicle immediately leaves - it can park, get ready and fire a full salvo in under a minute. When the first rocket is detected a couple of minutes later, the launcher will already have driven off and counter-battery coordinates will not be that useful/
Most of the death toll is women and children (7k and 10k, respectively). Even if you assume all men killed are Hamas fighters, which is not true, that is very high when compared to the attack which triggered the war.