The point of the illusions in a good magicians act is that they look like magic. If the illusions look like invisible strings/etc, then they were poorly done. Even if you know there must be a string, it shouldn't look that way.
So what I'm saying is that even if it's reasonable to deduce that this supposed magnet is being suspended from a string, it doesn't look like it is. If it is fake, it's a well-done illusion not a shoddy illusion.
Whether it "looks like it's on a string" is highly subjective. Suppose I primed you by saying: Look at this speck of dirt on a string, does it look like a speck of dirt on a string to you?
So what I'm saying is that even if it's reasonable to deduce that this supposed magnet is being suspended from a string, it doesn't look like it is. If it is fake, it's a well-done illusion not a shoddy illusion.