Van der Waals attractive force is a very powerful force but it only operates over a small distance. Significantly less than the repulsive force due to overlapping electric double layers. It is, however, very strong and if the cell can get close enough to the surface, the van der Waals force will hold it very tightly. Note that when the cell gets very close to the surface the van der Waals force starts to get very big indeed. In fact, at very small distances the van der Waals attractive force is enormously bigger than the electrostatic repulsion.
Bacteria are almost the same size as colloidal particles.
Modern thinking on how bacteria stick to surfaces and to each other is greatly influenced by the work done by two groups of researchers who worked independently of each other on the chemistry of colloids. They were interested to know how colloidal particles managed to stay suspended in a solution and the factors which caused them to precipitate. The workers in one of these groups were called Deryagin and Landau and in the other were called Verway and Overbeek. Between them they worked out the mathematics which described the forces acting on colloidal particles, some of which you have seen above. Their theory became known as the DLVO theory after the initials of their names.