What does Flash Magic do exactly with the Interface selection set on "None(ISP)" as opposed to "FDI USB-ICP-80C51ISP" when you are using the 80C51 programmer? It seems to still control the DTR and RTS lines at least with the "Assert DTR and RTS while COM Port open" selected in Advanced Options/Hardware Config.
Removes features from the UI that are not relevant.
Forces the use of DTR and RTS to enter and exit ISP mode.
Sets the post open delay to zero.