It’s a pithier way of saying, “What is true has always been true.” If it took 1800 years before someone came up with an idea (like Darby) then Christians have been believing a falsehood from the time of those who knew Jesus best until that new idea, or that new idea is wrong.
Sometimes what has been true isn’t always recognized.
The Jews missed Jesus and He walked right in their midst, fulfilling prophecy.
It may just be a matter of understanding.
If Darby said anything in the bible is wrong, then he’d be wrong.
But you misrepresent Darby. Christians had various favorite theories; fewer of those who held the theories elevated them to the level of basic faith upon Christ. Now not all the theories could be true, but some were nearer the truth than others. God in His grace will graciously correct the errors of a willing soul. I too have some theories, but I do not depend on nailing them all correctly for salvation.