With due respect, we are trying the answer of "..can I teorically connect a 2.1 Vf LED directly to a 2.1 voltage supply without any resistance? " Just providing proper values in the equation I = (V - Vf) / R will give us the answer. Your sticky also points to the same answer.
With all due respect, please go back and re-read the sticky again, you are only grasping part of the picture... The equation you point out is how to 'destroy' the excess voltage with a resistor and lower the voltage to the desired Vf, in this case we are already at Vf and have no 'excess' voltage to destroy... Plug the numbers in and you will see the equation fails to result in a valid answer if V and Vf are the same... 0.02 = (2.1-2.1) / R ---> 0.02 = 0/R ---> 0.02 = 0 aka failed value for said equation to work... When reading the sticky this time focus on the impedance of the LED at a given voltage, yes LEDs do have their own "resistance" it's just not linear, and this plays a role... Look at a forward current vs voltage chart of any LED...
Example: Lets call this a "3.5V white LED"
Looking at that chart below you can see that at 3.5V, the LED will pass give or take about 19mA due to it's own impedance (resistance) at 3.5V and 'room' temperature...
As long as everything remains in balance, aka the voltage doesn't drift and the temp doesn't rise or fall the LED will continue to pass just 19mA...