In the las manga, we saw him warping a tentacle, he JUST warped a tentacle, he didn't warp a whole bijuu.
He was capable of dragging out of the other timension a whole bijuu.
It's the same thing to warp a bijuu from inside that from outside.
If he was capable of kamui-him OUT, he is capable of kamui-him IN.
Proove me with manga that warping things out is easier for kakashi than warping things in.