I think he's talking about the gates that handle flights to/from Schengen countries being in a different part of the airport, before passport control. The assumption is that once you're in Schengen the border guards at the first entry point should have ensured you're allowed to be there.

However, in every airport where I've seen that, I've certainly been able to get as far as the gate without ID (particularly if I've a pre-printed boarding pass), but I've always needed either a passport or EU member state ID card to actually get on the plane.

If you can get on without your ID, however, there are generally no checks at all at the far end.

Ah, OK, this makes sense. I admit that I don't think much about immigration or customs when I am arriving in Europe because it is all so simple. It's only when arriving in the US that I dread the experience. (And I am a natural-born US citizen.)