It's the keymap that defines double-click to trigger FILE_OT_execute(), the operator shouldn't assume that's the case. Otherwise custom keymaps can break.
E.g. old Blender versions would open the active file when double-clicking empty space, some users may have set it up to do that still. There would be no way to do that anymore.

There are multiple things we could do. But I think it's best to separate the window level execute operator from the double-click execute operator. They'd share the same logic mostly. I already started looking into that, I can finish it if you want?