That list has more than a couple "Pro" DAC's on it (Grace, Antelope, Benchmark, Behringer). The Mytek is fairly new, and Stereophile takes it's sweet time reviewing products.
Unless you need the validation of having a product you own on someone else's list, it doesn't really matter.
RE: S/N's, them being in the 20K range does not necessarily mean they've produced 20K headphones. What companies will often do is start each batch in a totally different range. February's batch will all be 10K's, April's will be 12K, etc - this helps pinpoint issues if one batch was improperly made.
Then again, I would not be surprised if they've sold 20,000 headphones over the last 3+ years it's been on the market.