I don't know what his reason would be for the comparisons he mentioned, but mine would be the fact that they would all be miscast in those rolls, just like DiCaprio is miscast here.
DiCaprio may be a capable actor, but he's not believable in the role of Hoover. He just doesn't have the gravitas or stature as a person, not to mention age.
For this movie to work, you'd have to have someone like a Frank Langella, Christopher Plummer or Max Von Sydow, but a generation younger. I'm not exactly sure who that would be, but I know it's not DiCaprio.
It isn’t great casting, but the others are questionable actors, or not actors at all. Leo has the chops to play the part, if not the age or look.
A lot can be done with makeup though, look what they did to Charlize Theron. Too soon to know if that will be enough. I wouldn’t say Billy Crudup looks particularly like Hoover, but he did a good job in Public Enemies. He is about the same age as Hoover was when that film took place though, looks like Leo will be about 6 years younger than Hoover was at the youngest the film will concentrate on.