Tanks have more than one hatch. (IIRC a M60A3 has two on the turret and a driver hatch in the hull). He was mentioned has having cut padlocks on a few tanks, which supports the supposition they were locked in that way.
Once inside, he may have combat locked the hatch he had broken into, but not combat locked the other hatches, which would still have padlocks on the outside.
Again, that’s all speculation but based on the way combat locks work I have trouble picturing police somehow cutting the door itself open.