Every LED from each manufacturer has a different characteristic. I have found that LEDs will illuminate weaker and weaker as the voltage drops below their "characteristic" voltage. Many will still illuminate when the voltage drops by as much as 0.5v.
The tiny LEDs are extremely dim. Ordinary LEDs are 1000 times brighter.
You don't know if your LEDs need 3.2V or 3.8V so it is a gamble to see if they will light with only 3.3V.
The output of the micro-c will be less than 3.3V when it has an output of 10mA.
The max current for the LEDs is only 10mA, not 20mA.
To me that seems to be a high foreward voltage compared to typical LEDs. I have some that have a foreward voltage of 1.8V. LEDs are cheap, and you should be able to find a foreward voltage lower than 3.3V.