Depends on how strong the magnet is. Since its an enclosure and not a case, it should be fine. The reason magnets are not good is because of the interference it can cause with the electromagnetic field the Note screen generates when in use with the S Pen.
Someone demonstrated this interference using the S Pen. The the user drew straight lines until it began to distort and the straight lines began to bend/curve. This distortion occurred at the point where the magnet was placed on the back of the Note. However really weak magnets seem to have no effect on it.
I have a Note 10.1 that has 2 very weak magnets in the corners to keep the lid closed over the screen. When folded back to use the Note 10.1, it still works without any issue to date.