Anecdotal as well: It's all relative.

I was homeschooled for much of my childhood and first went to a public school my Junior year of high school. I didnt think of myself as exceptionally smart - average at best. I was in a English class one day half way through the year when the teacher was putting us into group. A girl he paired me up with said, "Yay he's smart!" and that was the first time I ever thought I was a "smart" kid. That was a town in Vermont where the public education system was not very good.

Later when I went to college I realized I was just about average at most things, a bit above average at some, and significantly below average at many things (due to poor homeschooling).

As an adult I realize that I'm more intelligent than the average joe but when I'm in a room of professors or even people who have a passion for studying and learning I can tell I'm simply not at the same level as them.

Impostor syndrome is not incompatible with being smart. Quite the contrary. It could be an extension of the Dunning-Kruger effect. The more you know and understand something, the more you can judge your performance and notice your failures, thus making you feel like an impostor.

Another anecdote: I finished high school 26 years ago. I too wrote O'Levels and A'Levels. A few things I can share on the classmates I keep in touch with or am aware of what they are up to. The most successful were not the smartest, smart as in top marks in maths which was the gold standard back then. The sentiment you expressed sometimes works in the opposite direction. Because my classmates knew they weren't the smartest, the worked harder in university and in life in general. Hence their success. The smart kids all "made it" but the not so smart kids 26 years later are much better off.

because both are self induced judgements ? some times you will have confidence (smart) and sometimes not (impostor).

The desire for adults to see genius children is detrimental I believe. I too grew up with lots of praise although it wasn't family only. It didn't help me much. Especially since I was kept in normal schools at normal pace, so now I'm bored and spoiled. College slapped 90% of that out of me. I still feel like a genius occasionally and although feel like a fraud most of the time.

to add some value to the topic of education, what people call intelligence in children is often an ease for some abstraction and systematic thinking. While most kids are scattered and down to earth. But there will be subjects or times when even your(my) nice brain will fail to appreciate some idea or detail. Especially outside of your natural tastes.