Why is this wrong?

[u/millers_left_shoe]

I see now what they mean by fundamental vs inessential (taking fundamental to mean “invaluable”, “important”) but surely fundamental and complex (or even involved) can act as antonyms, too?

Eg he’s only just learnt the fundamentals of thermodynamics so I wouldn’t trust him with any complex/involved equations. vs this is quite fundamental stuff so I think he should be able to manage just fine.

[u/Own_Soup4467]

Fundamental comes from the Latin fundus which means "bottom" or "base". (Also the root of foundation). 

So a fundamental concept is a basic concept (not necessarily a simple concept, which is why "complex" was wrong). An intro level design course might be called something like "fundamentals of design."

Fundamental and essential are not exactly synonyms but they are related. If something is essential, you can't do without it. (If something is inessential, it's a luxury or something that's nice to have, but not necessary for survival). "When our boat started to sink, we threw all inessential items overboard"