I think Siegler's main point wasn't aimed at site owners, but people who treat Alexa and Compete stats as an accurate representation of actual traffic only to then conclude things that in TC's case are apparently opposite of truth. I don't think anyone who owns a site will look at Alexa or Compete to see how much traffic the site is getting, only to see the general traffic rank among other sites.
Good points though about them still being somewhat useful for comparing flawed data with flawed data to get a general idea of how sites stack up to each other or whether there is any action going on at all. I guess it works if all said data is flawed in the same way. I'm still not sure how to explain it showing the opposite picture in TCs case. They seem to be wildly off.
I suppose it helps having as many possible sources of such data though. Are there any other sites like Alexa and Compete? Quantcast?