I'm like 99 percent sure the timing is correct.The only way there is 0 compression on all 4 cylinders is if the crankshaft gear is about 20-45° or so off from the cams or by some freak accident valves on every cylinder got hit. The latter would mean that intake and exhaust valves would be damaged. Doing a leak down is moot when no matter what the problem is being caused by, the engine would have to be removed and disassembled to correct it. As long as the timing is correct now.
There is another way, but since the engine was running prior to the belt popping off it's unlikely. The other way is valves are out of adjustment on the 4 cylinders and can't close or a good number of stem seals or seats is lodging them open. A leak down would tell you which is open on each cylinder, but that still leads to a tear down.
Are you sure the timing mark on the crank is correct?
The timing marks were all lined up and matched the marks on the belt even. I spun it a few times by hand and it still lined up.