That is only true of a registered receiver gun, and specifically one in which the receiver was not modified to accept a full auto lower before the registration. This is because in a registered receiver gun, the sear was 99.99% of the time not registered and had to be "married" to the existing lower when the gun was registered. Since the new-style Navy lower was not out when people were registering receivers, the ATF knows that you must have upgraded something. By doing so, you must have taken parts out. They consider those parts you removed (the old lower) to be a machinegun. Since you took them out, you have just made a second machinegun, which you cannot do, as that is manufacturing a machine gun after May 19, 1986.
This is not true of sear guns, which are 99% of HKs. My gun is a title-1 semi-auto in which I will fit a registered sear. It is ok to put a registered sear in a new-style lower and change parts at will. So I can use a Navy lower as can most people.
I am new to this, but I read that page in detail recently.