that sounds like a plot hole so let’s put some plot into it

a lot of technological advancements happen in times of war, that’s when scientists get the funding to make weapons, but that research doesn’t go to waste afterwards, the knowledge stays and can help in times of peace

what if the ship was sent, and a great war broke out soon after, all records and those who remembered it were destroyed, all whilst technology advanced becuase of the race to invent new weapons, and now the knowledge is being utilised for a more cheerful cause - space exploration, and FTL travel

the forgotten ship would peacefully sail across the great dark blissfully unaware of what happened, of how their mission has been erased from history, regardless if it was on purpose or accidentally. Their eventual arrival would make news across all human colonies, their language so ancient, their biology similar but in a strange way imperfect, living relics of the old world ripped right out of already faded pages in history, what a shock it would be for both sides of the encounter