You're apparently confusing absolute maximum ratings and specified operation conditions. No PIC is specified
to operate above 5.5V supply voltage. Keeping the voltage below 6.5V avoids permanent damage, but doesn't
guarantee correct operation.
The said HV types have an internal shunt regulator, but the supply voltage for correct operation is limited to 5.5V
as well. By adding a series resistor, you can power it from a higher voltage, bunt involving a lot of dissipated power.
Using a standard voltage regulator, e.g. 7805 is always preferable.