I am curious. "A species of guerilla warfare" is the term used by Captain Butler in 1905 ( don't see that the phrase is notably contemptuous).

As you say, the term tautologous 'guerilla warfare' is unlikely to have been used by Moore or Stuart as the spanish term only entered English parlance once the British began campaigning in Spain and the words guerilla and guerillero became conflated, by Wellington and other British officers, so that terms like 'guerilla fighter' and 'guerilla band' (e.g.) contracted into plain 'guerilla.'

Taking Napoleon's references of 'brigandage', 'robbery' and 'assassination' as an example, it seems that is how the class-based insurrection in Calabria was viewd by both French and British officers.

The British evidently had less reservations when the activities of brigands were directed agains the troops of an occupying force that as allies they were both working to defeat and expel. The situation becomes somewhat blurred when the activities of guerilla bands were directed against non-combatants, whether people who supported the wrong side or victims of brigandage plain and simple.