The motion itself [1] has zero reference to the Mueller investigation. It is in fact about Title 18 U.S.C. Section 2242b, which is states that

"Whoever [...] knowingly persuades, induces, entices, or coerces any individual who has not attained the age of 18 years, to engage in prostitution or any sexual activity for which any person can be charged with a criminal offense, or attempts to do so, shall be fined under this title and imprisoned not less than 10 years or for life."

The defendant in this case is not Assange, but obviously someone who has some sort of connection with Assange in this matter, as stated in paragraph 5 of the motion. Could it be that it is something just the get him and then slap some additional charges onto him? Sure. But this motion does not tell us that.

[1] https://pacer-documents.s3.amazonaws.com/179/399086/18919235...