Ukraine has not officially confirmed the strike, but one official told Kyiv Post they ‘stood in the open air for two hours’, long enough to 'transport the HIMARS, enter the coordinates and hit them.'
To be clear, they are saying Russia is furious with their military:
"Russian milbloggers have said their army is “at war with our own stupidity and sloppiness”, after reports emerged that as many as 100 of Moscow’s troops were killed when a Ukrainian strike struck a gathering of soldiers waiting for their commander to give a speech. "