Why a shared 533 mm tube does not guarantee compatibility

Are Spearfish and Mark 48 compatible simply because their launchers are 533 mm?

Not automatically; the physical tube may fit, but electronics and proprietary launch-system interfaces still require adaptation.

The physical tube dimension is only one part of the wider launch-system integration problem. A 533 mm weapon may fit the nominal diameter used by several European submarines, yet that does not make the surrounding system interchangeable. Electronic interfaces, fire-control arrangements, and proprietary equipment can still prevent a Spearfish or Mark 48 from functioning in another navy's launcher. The systems are adaptable rather than automatically compatible.