The rocket equation applies to all momentum machines which carry its own "fuel", that is, the mass to eject.

Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.

Agreed, but to be pedantic, it applies to all momentum machines which carry its own "propellant", not "fuel".

For instance, in a NERVA rocket, the "fuel" is the uranium rods in the reactor, the "propellant" is the liquid hydrogen that is heated and shot out the exhaust bell.

is there a form of transport that is _not_ limited by the rocket equation?

Solar Sail, Laser Sail, Electric Sail, Magnetic Sail, Mini-magnetospheric plasma sail, MagBeam, Plasma Magnet Sail, Photon Drive, Bussard Ramjet, Ram-Augmented Interstellar Rocket.

Basically any spacecraft that does not carry its own propellant. Most of these have either pathetic thrust or are way beyond our current state-of-the-art.

Solar sails, bussard ramjets, space elevators.

The rocket equation still applies, but the propellent velocity is so high that you don't a large mass fraction.