The "forgeries" claim has long since been dismissed. It was originally made by the later Hannovers on the British throne for obvious reasons, and the online Britannica articles are decades, if not centuries behind. But seriously, by the 1920s and Schnath's time it was already generally accepted that the letters were the real deal.

(BTW, Fritz and Ulrike had no doubt about that just a measly few decades after the event, either.)