The comparison simply means that Jem is starting to look more like Uncle Jack Finch who is fair, protective and brave and has an understanding that sometimes world can be unfair. Jem also possess these qualities and he likewise starts to see that there is not much to the myth of the Radleys and that Boo has the right to his privacy and a right to live as a recluse. He also starts understanding that whatever happened to make him a recluse must be terrible indeed. So by telling a strange tale to Scout he is protecting both Boo and Scout, as he is trying to protect Scout from realizing the truth.
