I think it's my first time ever seeing anyone online request someone else to not use idioms. Like a teacher correcting an essay or something.
(not /s)
(b) If you are a nonnative speaker, what's the difference between looking up an unfamiliar idiom and an unfamiliar word?
(c) Your pulled quote contains only two idioms. My best guess is that you're calling face value an idiom, but it is a transparent construction that means exactly what you'd expect if you knew the words face¹ and value.²
¹ https://www.merriam-webster.com/dictionary/face noun 5(a)(1)
² https://en.wiktionary.org/wiki/value noun 1; 8
(a) These are all common vocabulary items;
(b) If you are a native speaker who needs to look up a lot of words, what makes these different? ;
(c) face value is not an idiom. It is compositional.
What would you say if you saw a reference to someone as a "Svengali"? That is a literary allusion. It can't be an idiom because it's only one word long. Do you think it's an idiom anyway? In what sense?
It's possible to understand face value as metaphorically referring to the face value of a coin, or as directly referring to the value that any given object or event appears or purports to have on its face, but in either case it won't be an idiom.
> but excessive use of idioms makes it hard to understand what someone is actually trying to say
This is just false. From a communications or a vocabulary perspective, there is no difference between "idioms" and "words".