Sollozzo did not believe Tom was in the muscle end of the family. So, Sollozzo rightly believed.

Allowing for authorial license, I guess the Feds could have gone after several members of the Corleone family. Since the Feds were apparently surveilling the Corleones at least since the mid-forties at least, they probably knew of Tom's position in the family and certainly they did after the murder of the Dons.

I think it was just FFC's choice to ignore Tom's complicity as a component of the movie.