Looking for a word that captures knowledge, and falsely assuming that everyone else has it, thereby selling oneself short
Unconscious competence
Where a person has learned a skill and embodied it so much that they have forgotten that they ever learned it. They thus imagine that everyone else knows it as well - having forgotten the process that they themselves went through, to learn that skill.
https://en.m.wikipedia.org/wiki/Four_stages_of_competence