What he means is that every now and then there will be a scheduled change of equipment on a so-called through flight.... for example, there is (was) a direct one stop flight from BOS to JAX with a stop in ATL. The flight also changed equipment in ATL, from a 737-800 to a 757. Same flight number both legs, but in reality, a connection.

And yes, you can indeed miss the second leg if the inbound leg is significantly delayed!

Flights like DL 148 LAX-JFK-FCO have a change of equipment from a 757 to a 767-300 and so forth.