Here's the main theme inits original form. Click here to listen.
Here's "Across the Stars" in its original form. Click here to listen.
I do, however, hear rhythmic similarities, particularly in both themes' use of prominent triplets. To visually illustrate, here's an example with solid red lines showing exact rhythmic similarities and dotted red lines showing comparable similarities.
Thus, while "Across the Stars" has significant rhythmic similarities to the Star Wars main theme, it is clearly NOT an inversion of the main theme.