He means the nail could get infected which could be severe because it is inside you while pure external there's nothing inside you that would get infected.
In external there is the possibility of deep pin tract infections which are on average 1-2 per patient (almost all patient have infections), however the majority are superficial and easily treatable. Nonetheless they could go deeper.
In LON, because there is an external and internal device at the same time there is a greater risk of deep infections because there is contact from outside the skin (due to the pins) and the intramedullary nail which can be colonized with bacteria. The treatment in this case is removal of the nail and parenteral antibiotics for a prolonged time.
In LATN, the risk is lower because most of the times there is not concurrent internal and external device. Even so there is a greater risk than with pure internals. If a pin infection develops during lengthening in LATN is has to be treated prior to IM insertion. Even being careful, sometimes there is bacterial colonization but not yet infection so it could go pass unnoticed. Then with the IM insertion is could progress to a deep infection.
THE POINT OF MY POST IS PERMANENT KNEE PAIN!!! Forget LATN or LON