There's nothing special about composition that it requires language support, or that it has to be static for it to be considered composition. You can implement dynamic composition in OOP simply by having a List of components, and that's how many games that don't use ECS did and still do. Composition has absolutely nothing to do with inheritance or with requiring static data members.

Unlike you're claiming, the relationship between entities and components definitely does exist in ECS, just not in an OOP way, because ECS is not OOP (even though ECS can be implemented in any language).