The belt will stretch.

You have both cams locked, the crank at top dead center and locked by the flywheel and the fuel pump centered on the mark. When you tighten with the belt tensioner, you hope that there will be even belt tension all around.

Now, if you also hold the cam sprockets in place and they are slightly off, you will force the belt to be tight on one side and loose on the other. When you remove your locks, the belt will equalize tension and the cams will be out of alignment.