As a witness of people's character and to incidents, I'd always assume John Thorpe is unreliable, unless backed up by other evidence...(out of interest, in the novel does John ever give a reliable report? Maybe his remark on Gen. Tilney giving good dinners holds truth ?)
But my point is, even a mutton head like Thorpe should know better than to say Mr Allen had was a retired tradesman. It was not the occupation of a gentleman, and in the Regency, a newly minted fortune did not hold the same allure as old money.
As Thorpe is attempting to impress General Tilney that Catherine is heiress of a great fortune, he'd unnecessarily risk putting off any gentleman of old family to mention a fortune from trade.
Admittedly, I haven't seen NA2 (or any JA adaptation for ages ). Have fun watching the film, but when you check out NA2, does the film anywhere correct the impression Mr Allen made a fortune from trade ?