It's not clear what's going on with the LED in the article's schematic, because only one side of it is connected. Depending on how the other side is tied off -- you can tie it to VDD or to ground -- you can get "lit high" or "lit low" behavior out of it. (And, of course, the polarity might be reversed. LEDs are notorious industry-wide for having confusing polarity markings. So never, ever assume a LED is the right way around....) That's because this NAND gate is quite good at either sinking or sourcing current: 24 mA either way (at 3.3V). Older designs are not so balanced, or capable of so much current. (The LVC family is often chosen for exactly this reason.)