I think Roger and co simply had their own timeline/prediction.
Which was wrong though.
There are at least 4 foretold people who need to be born in the same era for the world to change.
The sea kings told us about their two sovereigns, one being Shira, the other one likely Loki.
The Roger Pirates and MJ know something about Shanks that we don't yet, but he wasn't called a child of destiny for no reason.
And a person who can awaken the Nika fruit is also needed. Roger thought that his son could fulfill this role; Ace failed, but Luffy succeeded within the same timeframe, so the Roger Pirates' estimate was not wrong.
On top of it, a wise Wano ruler seems to be added to the equation, otherwise the WG could become unstoppable.
IMO, these chosen ones come in pairs.
Shira and Loki as non-human leaders.
Shanks and Momo as rulers who facilitate the dawn.
Luffy and BB as dawn-bringers, one through liberation, the other through destruction.
When all these six are alive in the same age, fate can upend the world.