Hopefully in the past year you've figured out how to remember this, but in case anybody else is reading along...
Think of the word "beverage". Etymologically it comes from "bever" (which means trinki) and a suffix very similar to -aĵo.
> is food made from meals or is a meal made from food?
Neither! The part of the equation that you're missing is the word manĝi.
So, starting with the verb manĝi we derive manĝo. Basically it's the act of eating, but traditionally is understood as the act of eating a meal.
So just like "beverage" above (literally "drinkage"), we can form "eatage". Manĝaĵo is also formed from the verb (manĝi) and not the noun (manĝo).
OK, let me add my thoughts on this. First, apologies if this was clarified in the above-mentioned Facebook group. I don't have a Facebook account (and don't want to have one) and can't check what people wrote in the group.
Second, we should realize that the English word 'meal' can mean at least two things: 1. the time/activity when people sit down and have breakfast, lunch.. 2. the actual food you eat during such an activity So let's refer to these meanings as meal(1) and meal(2).
My understanding from salivanto's answer is that manĝo is the act of eating therefore coincides with meal(1). One can make statements like 'lunch is my favorite meal of the day.' But the sentence 'Esperanto speakers buy meal', in my opinion, suggests that those speakers are buying actual food not a service of eating therefore I think the word meal there means meal(2) and should be translated as manĝaĵo. One can point to the food that you are eating during a meal and say: 'I don't like this meal.' But that is not supposed to mean that you don't like having a lunch but that you don't like this concrete food that you bought.
Any thoughts on that?
I had wrote there https://www.facebook.com/groups/duolingo.esperanto.learners/permalink/589745027854185/ about the meal vs food and they had said:
"Mangxo is a meal, trinko is a drink. Mangxajxo is food, trinkajxo is drinks. Definitely different concepts."
I disagree with the answer you're quoting - and if you look at the other replies in the same thread, it seems that others disagree as well. I'll point out that the second and third answers on the thread are good answers and are written by experienced Esperanto teachers - one of them a member of the Akademio.
My summary: trinko is not usually used, but would mean an instance of the act of drinking. (We sat down and had a drink together.) You'd probably say "ni trinkis". Manĝaĵo is indeed food, but trinkaĵo is usually a single beverage.
That would be "Esperantist."
The online dictionaries seem to agree that this word in English simply means "speaker of" or "specialist in" Esperanto. It can also mean "related to Esperanto." I find that people who don't read dictionaries often assume it means "proponent of Esperanto" - which is why it's often translated simply as "speaker of Esperanto" to avoid that misunderstanding.
I insist in what I have already said about other sentences: if "an Esperantist" (or "an apple", "a car, etc.) is accepted as a translation for "Esperantisto" (or "pomo", or "auxto"...), i.e. no article in EO -> indefinite article in EN, then "some Esperantists" or "a few Esperantists" should be accepted as a translation for "Esperantistoj", since some/a few is the de facto plural of a/an in English.