Almost looks like surface rust just kept developing until hole stage occured. Possible originally started with poor prep work and paint adhesion when car was manufactured. Hard to say but not unusual. Seems like moisture gets behind and almost lifts paint off a mercedes ahead of the rust. Not typical of most brands of cars. Seems to be very poor adhesion of original primer or perhaps a non waterproof primer was used during production. What I religiously do after repairs are complete and painted is to inject by air pressure and small canister type sprayer a good dose of oil into the rocker panels incorporating some grease as well in the mix..As i live far north there are always 1/2 inch plastic disks to remove on bottom of rocker panels to apply the mix. Even if not there you can buy the plugs that seal a half inch hole at some auto parts store after drilling the holes for less than .10 each. Just a standard undercoating accessory. Also allows you to drill additional holes and cap them if you do not think you are getting a particular area covered. Nothing basically stops rust totally except it's complete elimination in my opinion. But the mix of oil and grease will reduce the activity level to a much more manageable level and help retain your structural integrity and especially the jack points. I personally estimate the factor is at least 50 to 1 if the trouble is taken to get total coverage inside the rocker panel. Over the years have spoke to corrosion engineers and they concur. The oil and grease just seem to seep all around in there and cover the oxide and metal. This lessens the amount of oxygen available and slows down the process by an enormous factor. An additional benifit is it helps reject the moisture that accumulates in the rocker by acting as a barrier layer between the water and metal. No other type of undercoat over rust has this type of performance in my opinion again. In fact some seem to even accelerate the oxidation process. At least it's good you did not use plain body fill as it retains moisture and is not waterproof. That almost guarantees paint bubbles down the road if you stuff body fill into a hole. The fillers like short hair fibreglass fill are the proper item to use in areas like this as they are waterproof and economical. A little harder to work depending on what tools you have available though. What you have used has simular qualities. Oh yes, when i have a spot like this i just use a spot blaster to get rid of any rust, metal prep the small area, do the fill and leveling work, prime and paint. But most important protect what is behind the repair if possible. Mandatory on new car repair i believe. If one does not own a spot blaster just drive car to someone that has as only takes a second or two to do it and then you are not burying rust under your body work and that is important. Take most miracle claims with a grain of salt as even myself wish they really worked long term. Just one persons opinion though and not even a body shop guy. Just do my own work.