It's important to distinguish between the two different types of "crisping" that both happen in bread.
The first is the Maillard reaction which is caused by the sugars reacting with proteins; this is facilitated by high heat and low moisture, and is what actually causes the bread to turn brown (and eventually to burn).
The other is simply the evaporation of water - drying it out - which can make the bread or crust noticeably "crisper" without any browning.
The Maillard reaction happens at 154° C / 310° F, which is much higher than the boiling point of water (100° C / 212° F), so the evaporation happens first. If you put a piece of bread in the oven at a low temperature and leave it there for half an hour, it will crisp up significantly but not brown.
So essentially it depends on what kind of "crispiness" you want. High heat will cause the Maillard reaction to occur and that will crisp it up faster, but you have to shorten the cooking time, otherwise it will burn. Lower heat, on the other hand, will crisp much slower - and if the heat is too low, you won't get any browning - but you can leave it in there for much longer and the crust will keep getting drier (i.e. crispier) due to the water evaporation.
The instructions are correct. High heat causes more "crispiness" in some applications, where almost all of the crisping comes from the Maillard reaction or caramelization of some kind, but bread is an exception because of its porousness and high water content (easy for water to evaporate).