Async and "going directly to I2S" are not really related to each other, though they are both parts of USB to DAC connection. As the Wavelength link explained, the Async bit is all about where the clock for the DAC is generated. In adaptive it comes from a filtered version of the avearge timing on the USB bus and hence can be very noisey. In Async the clock is next to the DAC and the USB has to keep time with it (with a big buffer to allow for it to miss a few times and not screw any thing up).
The directly to I2S is about what happens after the digital audio data is reconstructed from the USB data stream. Most TI 270X chip based solutions, most of the USB DACs out there, get a normal SP/DIF signal from the USB converter (270x) and feed that in to a reciever chip, along with the other inputs the DAC accepts, and that receiver chip outputs I2S to the actual DAC. As the wavelength DAC only have one input, they don't need a reciever chip, and have the USB converter output I2S directly to the DAC chip. However, many versions of the 270x can also do I2S directly, the manufacturers just don't use then that way.
Hope that helps.