When you say continuous interlocking U shape, are you saying it fills one channel from the top until the connected channel fills from the bottom?
A solver pairs the triangle with one of its neighbors. Then cuts a window at the bottom during the slice. So plastic gets injected into the top of a triangle tube and flows through the window at the bottom of the U to its neighbor cell.
Download the binaries and try slicing with magma infill type. Turn off visibility of all the other line types and you'll see how it works