Asus forked Tomato back in 2010-2011 to build Asuswrt off it. Since then, the vast majority of the code has changed so much, the two have almost nothing left in common. Asuswrt's kernel for instance is more tightly dependent on precompiled kernel modules from Trend Micro and Tuxera, meaning that making any change to the kernel carries a risk of breaking compatibility with these modules, as they would generate kernel symbols or structure definition changes.
