Detecting "arbitrary program + shell" is at least moderately more difficult.
It's the attacker's dilemma though. They only need to trip one alarm to trigger IR.
Notice that they didn't actually have an alert for Firefox+Shell, they detected that later by inspecting the audit logs.
They explicitly state it was one of the behaviors they detected as suspicious.
Detecting "arbitrary program + shell" is at least moderately more difficult.
It's the attacker's dilemma though. They only need to trip one alarm to trigger IR.