Silver isn't using any models here (it's too early), he's just engaging in punditry, which is why he's been wrong about Trump at every turn. Nate is great with numbers, not so much at being a talking head. There's little reason to pay attention to Nate Silver this early in the race.
Every turn? There have been zero turns so far. We're still in the long void before people start voting. Nothing has happened, and Trump basically sits where he was several months ago.
Having a historical perspective is valuable and can at least inform rough likelihoods. His arguments that...
1) Polls mean very little months before primary voting.
2) It is extraordinarily rare for somebody outside a party to win its nomination.
3) It is extraordinarily rare for somebody with 0.0 political experience to win a major party's nomination.
4) Endorsements of sitting politicians in a party are one of the best predictors for eventual success, and Trump has none.
... are all sound, and should give some idea of what Trump's actual position is. He has no political experience, isn't a real member of the party, has no endorsements or support from the party, and his position in the polls and complete lack of upward movement despite the tightening field should all give some perspective.