Yes, you read it write, Explore M3 will not work with mbed platform. I had started an conversation with mbed long back when we started the project. The mbed Board that uses LPC1768 uses a custom ASIC for programming the board. Explore M3 eliminates the need for using that ASIC so that people looking to make custom boards, don't have to look for that unknown chip.

The emails take 2-3 weeks to reply, I completely understand with kind of breadth the platform has. However, when we get a reply its full of corporate mailing junk with no real conversation. Hence we decided not to pursue it further.