That thick of filler generally means it's covering deformities from a wreck that weren't/couldn't be worked back out. The rust doesn't look too extensive, so hopefully they just used that much (too much) to cover the hole and blend it in somehow. It's almost as if they mixed up a whole can and smeared it over the area for no good reason.
I've troweled what seemed to be quite a bit of filler on cars when I worked at a body shop, but by the time it was sanded and feathered in, it was never any thicker than 1/8" at most. What I see in the picture is nearing 1/4" or more. Even like when you see the guys on Boyd Coddington's old TV show glazing a whole car, the finish sanded result isn't any thicker than it would be if they had used a filler primer. Filler is more stable up to a certain thickness where it becomes a matter of preference as to whether lay down lots of primer to block sand or filler. Generally, you want to keep glazing putty and primer to an absolute minimum mil as it doesn't hold up. I've seen glazing putty flake off a well prepped fender that got re-dinged.
The problem with just blending the filler back in is that you don't know how stable the old stuff is, and could end up cracking through your new paint or having some other reaction. Keep in mind, any welding, even if it doesn't visibly burn/blister filler near the repair, it will separate the filler from the metal due to the heat sink. I'd get some 40 grit on a D/A and try to feather that stuff back as far as I could, hopefully it tapers off before too far.
