For hacking a casino? A private business unrelated to any US domestic or foreign interests?
Not a chance in hell it would be an act of war. Businesses get hacked by China, Russia, North Korea, and Iran all the time. Hell, China hacked the US Office of Personnel Management and stole the security clearance records for 22 million people in 2015 and even that wasn’t declared an act of war.
If an adversarial government hacking the US military and stealing security clearance records isn’t an act of war, a bunch of rich mobsters having their casinos hacked sure as shit ain’t.