He's completely right, the origin of "breakfast" is ending your nightly fast. It's been in use since at least the 15th century, prior to that (Old English) morgenmete was used; morning meal.

The same logic happens with the Portuguese word "desjejum", which can be broken in almost the same manner, meaning "that [meal] which breaks your fast (des+jejum).

Interestingly, in french, it's the same kind of roots (déjeuner, dé- (quite like the un- prefix in english), jeuner (fast)), but the word doesn't actually point to the morning meal, but instead, it points to lunch. The word for the morning meal is "petit déjeuner", literally "small un-fast". Presumably, people were not having a morning meal a long time ago.

Some compound words do, however, like firetruck. It's a truck. For fire. Breakfast falls into that category.

So then when McDonald's introduced breakfast anytime and had ads about having breakfast for lunch and breakfast for dinner they were suggesting fasting all day until you got an egg mcmuffin for lunch or your end of day meal?

I don't disagree with the it started as first meal of the day regardless of time of day, but these days it is more likely to mean morning meal than it is first meal of the day.

There are many ideas conveyed by a single word in English.

You could refer to the concept of breakfast, the meal, and also the common food of breakfast, bacon and eggs, with the same word.

Well it's sort of more of a watertruck really, isn't it?

It does not carry water but it does carry equipment to handle fire.

Firetrucks carry water.

Some do, others carry a large pump and hook up to a fire hydrant for the actual water supply.