Yes, it's a good fit. I agree it looks like it's not all the way through, but it looks/feels like the correct taper and there are just enough threads to fully engage the locknut. I don't think it's a problem. You want to clamp on the taper so there is no play. If the bottom hex sat flush against the bottom of the spindle arm, the taper might not fit tight enough and it might be more difficult to remove since there's nowhere to stick a pickle fork.
All that said, I'm not sure it's long enough to eliminate the bump steer, but it's already so long, I feel like it'd be smart to add some structure to the spindle. Since I have more time than money right now (and I have very little time), I might go ahead and do something like that. The problem with fabricating something, like Mechie points out, is it probably needs to be hardened to some standard spec. Grade 5 would probably be fine. The risk of the torch/oil at home would be making it too brittle and having it crack.