Why is LED lighting up despite Supply voltage < Forward voltage
"Typical" in datasheets doesn't mean anything useful.  Those are mostly marketing numbers and usually vendors trying to make themselves look good.

The min and max specs are what matter.  It's not surprising that a LED that typically has 3.6 V accross it at its full operating current would light up somewhat at 3.3 V.  The current is probably considerably less than the full current, but some LEDs are so bright that they are still easily visible on your bench at a small fraction of the maximum current.

No, this model LED will not reliably light from 3.3 volts.  You found one that did, and the next 1000 you get might also, but the next 10000 after that might be too dim.  Unless the spec sheet explicitly tells you what you get at 3.3 V, you have to assume there is no guarantee.  In reality you will probably get some light at 3.3 V, but the amount of that light could easily vary widely from part to part.