As a non-engine expert I can't say I have a problem with the desciption, there are after all 12 cylinders aranged in in two banks of six - horizontally opposed.
Indeed 'horizontally opposed type' is how the engine is described both in the Churchill Service Instruction Books (MkIII/IV, V/VII and MkVII/VIII) and by Vauxhall in their account of the engine design. The Service Instruction books also make mention that the 'flat twelve layout results in smooth runinng' so it would seem that both terms were generally applied to the Churchill engine.