Tom,
Daniel certainly has more information than I do to support that view. I'm only speculating based on aerials from the late 30s that I've seen - and what I've seen of Tull's solo work.
Emmet's work that pre-dates Tull's joining him in 1924 does show an evolution in style. One of the earliest I've seen is Cherry Valley (1916). The bunkers there look very primitive - long, thin, and in some cases perpindicular to the line of play. But moving ahead to Rockaway HC (1919) and Old Westbury, the style becomes similar to Seawane and Pomonok (1921) with the curlicue outlines, natural shapes and frequent use of mounding within the bunkers. His work at Nassau (1927 with Tull assisting) looks like a natural extension of his earlier bunker work done before Tull.
There are two Tull courses around DC built in the eary 50s. I can't see any similarity between them and any of the Emmet/Tull courses. Perhaps Tull moved away from the Emmet style in the 50s towards something "modern." But I'd feel more confident that he influenced Emmet if there were courses in that style that he designed on his own.