Repairing crack in interior block wall
I disagree with your uncle on the hydraulic cement part. Hydraulic cement when used properly has the viscosity more like water and is used to flow into cracks and crevices and harden which makes it hard to do on the side of a wall as it will just run down the block. In addition it is very hard when dried so eventually other cracks will appear on both sides of the crack you tried to fix in the future, because the block is weaker than the hydraulic cement.

The best approach is to place a backer rod which is like a mini pool noodle in the crack and then use elastomeric concrete crack filler, its like chaulking for concrete. The benefits are that it will bind to the concrete and if there is any movement in the future it will not crack and yes there will be movement however minor.
After you seal the joint then follow Uncles advice.