Depends how far below the other Hakaishin you believe Sidra is and if Whis was referring to Jiren's current power being > Belmod or Beerus or if he were estimating Gainz Bro's full power, since initial Jiren being the strongest Ki

ever sensed places him above both these Saiyans.
I'd go with placing Sidra a few leagues above his peers due being intimidated by Freeza and having a greater reaction to SSB Goku's power than the other Gods suggesting SSB tier aren't absolute ants to him. He gets wrecked by Vegetto alone.
Manga-wise, he'd still get wrecked by Vegetto if he gets hit, but may be able to outlast the SSB fusion's time limit due to being implied to have better defensive Ki techniques than the other Hakaishin.