https://forum.45homelab.com/t/motherboard-compatibility/858
There is a little extra room in the HL15 for some XL consumer boards, but not a 12x13 dual CPU Supermicro board. The larger boards may also have nonstandard standoff locations. I assume since @BVD is referencing Supermicro chassis, they are probably thinking 12x13 EATX.