That's the point aswell. If she were considered equals with those other legends back then, Oda would’ve mentioned her alongside the others. It's not like the Yonkou system was even around. Instead, she was conveniently still in the process of building her empire after the era of Roger, Whitebeard, and Shiki. Even later on, she viewed herself and her empire as beneath that of the old Whitebeard, aiming to surpass him by acquiring Elbaf.
If there is a pecking order among the old legends in their prime Shiki is definitely above Big Mom in that order, we can argue the gap is large or small but there's little argument for Shiki to be below BM when their primes aligned.