The misconception is mixing up notations. Using Big-O for the upper bound has been the norm for describing algorithms for at least half a century.
In Knuth's description[1] of Big-O notation (and related variants), from 1976, he starts out by saying:
Most of us have gotten accustomed to the idea of using the notation O(f(n)) to stand for any function whose magnitude is upper-bounded by a constant times f(n) , for all large n .
Emphasis on upper-bounded. He goes on to describing other notations: Big-omega, for a lower bound, and so forth.
In Knuth's description[1] of Big-O notation (and related variants), from 1976, he starts out by saying:
Emphasis on upper-bounded. He goes on to describing other notations: Big-omega, for a lower bound, and so forth.[1]:https://danluu.com/knuth-big-o.pdf