The above statement can be modified for better understanding. The nuclear force is the strongest force only in short range and the gravitational force is a strong force only in the large range, as in the case of astronomical level.

Let us elaborate on the above statement.

Nuclear force - The nuclear force which essentially holds the nucleus of an atom together is the strongest force has a range of 10⁻¹⁵ metres.  The strong nuclear force is the strongest at short distances within the nucleus of the atom.

Gravitational force - Gravity has an infinite range and is a purely attractive force.

If you take two protons as in the case of Helium and hold them very close together, they will exert several forces on each other. Because the protons have mass, the two protons exert gravitational attraction on each other. Because they both have a positive electric charge, they both exert electromagnetic repulsion on each other. Also, because of the short range between the helium atoms, there is attraction via the strong nuclear force. The strong nuclear force is the strongest at short distances, it dominates over the other forces and the two protons become bound, forming a helium nucleus. Gravitational force is so weak here that it is negligible. The nuclear forces are very short ranged that outside the nucleus the value falls off to zero.

On astronomical scales, gravity does dominate over the other forces. The gravity has a long range.  The earth and sun are far too distant from each other (billions of meters). Still  the earth held in orbit around the sun by gravitational force only. Though gravity is the weakest of the forces in general, but it is the dominant one at astronomical scales.