How much current can a MAX II I/O pin safely sink? 16 or 55mA?
I believe the graph shows maximum total current for all pins.
The datasheet is clear that 25mA is the maximum allowed per pin.
What the 16mA max in the table denotes is the maximum current the output pin can sink and still maintain a valid Vol logic level for 3.3V logic (0.8V).
But for an LED driver you don’t need to maintain a valid logic level.
It just means if you draw 20mA the voltage at the output pin may slightly exceed 0.8V.
Just take that into account in your resistor calculation.