Not to speak for Jake,

On this piece, 4043 would probably (most likely) have worked as well. 4043 is an easier to use silicone based filler and a lot of aluminum casts are silicone as well (because it's easier for the parts to release when they're made). 4043 is not quite as as strong. With the location of this bracket it will not see any real high temps so 5356 was the better choice (IMHO) based on strength. 4043 handles higher temps better, but the location and use of the part 5356 was a good choice.

Or, after all that rambling, maybe Jake had no 4043 filler?