This is really confusing topic for most of us, so I am hoping this explanation helps. First thing to remember is that anteversion and antetorsion are different. Antetorsion is twisting along the femur so that the head/neck is positioned more forward compared to the femoral condyles (I tend to imagine the head/neck in one plane and the femoral condyles in a different plane, with the ability of these planes to reorient based on the amount of torsion in the shaft). Anteversion is not a rotation of the bone, but rather how the femoral head (and, therefore, neck/shaft) is positioned in the acetabulum relative to a plane (I tend to imagine the head/neck/shaft moving as 1 unit). Individuals can have issues with either or both.
In the infant, they have about 30 degrees of antetorsion, which would result in IR of the thigh. They also have about 60 degrees of anteversion, which would result in ER of the thigh. These 2 forces are in opposite directions, but don’t cancel each other out because the ER is about 30 degrees greater; so, the net result is about 30 of ER. Campbell’s 5th edition, pp 104-106 does a pretty good job of explaining this. Fig 5.7 shows the newborn hip- you can see that the thigh is ER because the femoral head sits in more of an anteriorly facing position (anteversion) compared to the other pictures.