This is illegal by the current wording of the rule 3.5.5.
Releases have to be braced, or they have to fit one of two categories listed as the exceptions in 3.5.5.b.
1) 1. A non-braced top person in a vertical position at prep level may be released to a stunt at any level provided the top person remains vertical.
2) 2. A non-braced top person in a cradle position or horizontal position at prep level or below may be released to a loading position or stunt prep level or below.
Unfortunately, the seated straddle is neither vertical nor horizontal so it doesn’t fit into either of those exceptions.
They could take the seated straddle up to a prep and bump and go from there to the full twist to load and that would be legal.