In the midwestern USA, the meal in the middle of the day is very often called "dinner". For the people who use that term in that manner, vespermangxo is "supper".
Using those cross-terms would be too confusing, so the course sticks with "breakfast-lunch-dinner". As you can see, there's no such issue in Esperanto, who uses the times of day to point out the respective meals.
What is the difference between "nun" and "baldaŭ"?
nun is "now"; baldaŭ is "soon"
thank goodness, I'm starving!
Could you use "tuj" here, or would it have a different meaning?
"Tuj" is "immediately", so it conveys closer proximity in time than "baldaŭ" (soon).