Netflix is not trying to grow their subscription base.

The "risk" to which he was referring to is the risk of the show failing completely by some metric (e.g. no one currently subscribed to netflix is watching it), and all that money being effectively flushed down the drain.

It's the risk of going overbudget. Or the risk of actors and directors having a falling out.

There's dozens of factors that go into making a show successful and this test is to see how Netflix as a company can handle them--either directly or by proxy. If this test is reasonably successful then maybe in the future, they will make a play to have original content garner them more subscribers as a competitor/partner to say HBO or Showtime.