I am suspecting that the grammar system used to analyse French, that is taught in France (and maybe Québec and other French-speaking regions) has a slightly different framework than the grammar system(s) used to analyse French that are taught to English speakers. For example, when I see words like Substantif and Complément d'Object Indirect, I get a vague feeling that these are slightly different than the ideas of Noun Phrase or Indirect Object.
Here are two frameworks about French grammar that disagree with each other, that are taught to English speakers.
1) Thoughtco's View: Indirect Objects (that aren't indirect object pronouns) start with à, or sometimes pour, and never any other preposition.
All of the five or six websites I looked at last night seem take this view. For example, here are quotations from thoughtco.com webpage:
Thoughtco says that person/things preceded by de cannot be replaced by an object pronoun:
When deciding between direct and indirect objects, the general rule is that if the person or thing is preceded by the preposition à or pour, that person/thing is an indirect object. If it's not preceded by a preposition, it is a direct object. If it's preceded by any other preposition, it can't be replaced by an object pronoun. (https://www.thoughtco.com/french-indirect-objects-1368865)
And that the only two types of object pronouns are Indirect Object Pronouns and Direct Object Pronouns:
Object Pronouns in French There are two types of object pronouns:
Direct object pronouns (pronoms objets directs) replace the people or things that receive the action of the verb in a sentence.
Indirect object pronouns (pronoms objets indirects) replace the people in a sentence to/for whom the action of the verb occurs. (https://www.thoughtco.com/french-object-pronouns-1368886)
And that the indirect object pronouns do not include "en":
The French direct object pronouns are:
Me / m' –> me Te / t' –> you Le / l' –> him, it La / l' –> her, it Nous –> us Vous –> you Les –> them
The French indirect object pronouns are:
me / m' me te / t' you lui him, her nous us
vous you leur them
Since "de + noun" is not an indirect object in their framework, and since "en" is not an indirect object pronoun in their framework, what do they consider the pronoun en to be? Answer: They consider en to be an Adverbial Pronoun. (In their framework, the pronouns are: Indirect Object Pronouns, Direct Object Pronouns, Adverbial Pronouns, and Reflexive Pronouns.)
Adverbial Pronouns In addition, adverbial pronouns work in conjunction with the object pronouns:
Y replaces à (or another preposition of place) + noun
En replaces de + noun
Reflexive Pronouns Reflexive pronouns also come into play, particularly when trying to figure out word order for double object pronouns. (https://www.thoughtco.com/french-object-pronouns-1368886)
So, according to this framework, "de + noun" is not an indirect object, and cannot be replaced by an object pronoun; it is instead replaced with the adverbial pronoun en.
2) A certain textbook's view: Indirect Objects sometimes start with de, and the pronoun en that can replace them is indeed called an indirect object pronoun.
(The following are quotations from "Advanced French Grammar" by Monique L'Huiller.)
This framework does consider y and en to be indirect object pronouns:
(page 494, chapter "31 Personal Pronouns", section 2.2.1)
In the following quotation, "de + noun" is referred to as being an indirect object, and "en" as a pronoun that replaces that indirect object:
(p.502, "Chapter 31 Personal pronouns", Section 220.127.116.11)
(Bonus evidence? : The following is a comment from a WordReference forum thread, that takes a similar view as the textbook):
The following quotation is one of the few places on the internet that considers "de + noun" to be an indirect object:
A verb is indirect transitive if it takes an indirect object. An indirect object is an object that is preceded by a preposition (especially à and de, but also sur, dans, en, avec, etc.) (https://forum.wordreference.com/threads/fr-verbe-transitif-indirect.570123/)
According to the grammar system taught in the Francophonie, is "de + noun" an indirect object? If so, do you have a category of pronouns called "Adverbial Pronouns"?