I think you have the wrong definition.
There are two formal definitions
*lacking knowledge, information, or awareness about a particular thing.
*lacking knowledge or awareness in general; uneducated or unsophisticated.
The slang definition or informal one is based of the inclusion of uneducated or unsophisticated as a concept. Some people use the word rude to also mean rudimentary (basic) which also led to the concept of being discourteous. EDIT - it goes back to peasants being stereotyped as ignorant (uneducated), rude (lacking proper manners), and basic (lacking fashion, good housing, tools, etc.) It is not something most people do though -in that it is not polite to boil down lack of education into also meaning a person lacks manners.
Don’t try to mix the slang/informal definition with the real ones. You were ignorant of the trust levels - but having a link button you can’t use, is not treating you rudely.
He googled “Ignorant definition” and it comes up in the search index as the Informal definition of the word - (slang). He did not click on the link or do any more research into the actual word or what the real meanings are. No actual dictionary website was consulted.
EDIT
they are in no way upfront about how it works.
There is a Sticky thread explaining Trust Levels and what you can do with them at the top of the General Forums. New Forum Guide - Syntax and Features Your lack of knowledge on the forum features is due to your lack of reading the provided information.