I was actually confused by this as well so I started playing without crossing the thumbs. It seemed to go easeir for me after that and I fail to see how crossing your thumbs could make the piece easier.
Given that Rachmaninoff was very touchy about his music I assumed that the hands crossing was to mask the simplicity of the opening measures.
I would pay good money to hear someone play what Rachmaninoff actually wrote in this piece
You would pay good money to hear it how he wrote it, eh? Well I've heard a recording of him playing it and it's not how it's written. The first page is 'lento' ok, slow - but not when he plays it. By far the best pianist playing his stuff is Howard Shelley. Don't get me wrong, Rachmaninoff's playing is phenominal, and the way he brings out inner melodies is very unique. But his playing is also very dated and rushed.