Definitely chain tensioner. Chain tensioner has a one way ratchet that allows it to take up slack and not release tension.

You need to remove the tensioner before you can remove chain/cam gears.

I would say your mechanic has removed and refitted the tensioner as an assembly and hasn't reset the plunger.

Once you have timed the eng you would then fit the tensioner body and torque to 80 Nm, then the plunger, spring and screwplug torqued to 40 Nm.

The engine isn't a free spinner, so unfortunately the head will need to be removed again as the valves will definitely be damaged .

I believe this is 100% mechanics fault and he should pay for all repairs.

Good luck