You can demonstrate it somewhat empirically: pip, uv, etc. all understand the build backend interface and packaging standards and can install any package that conforms with the standards.

(I think so much of the confusion around Python packaging comes down the layer of abstraction/standardization not being the one that users intuitively expect. This is unfortunate and is made worse by lackluster user-facing documentation. But there are standards that guarantee interoperability, and they’re pretty close to Pareto-optimal in terms of making all involved parties happy.)