Do steam turbines define a dreadnought?

Would a 21-knot battleship with triple-expansion engines still count as a dreadnought?

Yes. The all-big-gun arrangement defines the type, while turbines provide much better sustained high-speed endurance.

An all-big-gun battleship can still be classed as a dreadnought with triple-expansion engines because several early ships of the type did without turbines. A 21-knot trial speed does not mean that reciprocating machinery can sustain high speed as long as a turbine plant. That endurance difference matters operationally even when both ships share the same classification. Propulsion changes what a dreadnought can do over time and distance, while its battery arrangement still defines the type.