exactly, OSM is meant to be based on original surveys rather than external sources, and something like "knowledge" or "local knowledge" is a perfectly fine thing to indicate as a source if you mean "I'm familiar with the area and am very confident that my changes are correct although I have not specifically surveyed it just to make these changes".
I know a guy who did his course and has his certificate of knowledge, that he printed out, framed and hanging on his wall, he always pulls it down to talk about it like it's a real thing.
It literally just says certificate of knowledge and made to look like a diploma. It's the second most useless thing in his house.
Personal knowledge can provide a level of detail in mapping simply unavailable elsewhere.