Yup, the sheep don’t read the books, do they?!

The lamb won’t drown until it tries to breathe, and it’s got no reason to try to breathe until the cord is broken. So whilst it’s in the process of being born, it does no harm to leave the bag intact.
There’s a possible slight exception to that, if a massive lamb is stuck in the birth canal and the cord is constricted. So if I went to a ewe and found it all jammed up, I might then break the bag - but tbh, in those circs the lamb probably has its chest so constricted it wouldn’t be able to inflate its lungs anyway.
I don’t think you do any harm breaking the bag; if you have to assist, the bag often breaks anyway. But yes, it gets less slippy and friction can increase once the bag has burst.