Because either Rey’s parents were on that departing ship or they’re dead and buried on Jakku. It can’t be both.
And Kylo can’t be trusted.
It can be both. The ship we see leaving doesn’t get high enough to be in orbit or leave the planet. It may not even be a spaceship or spaceworthy. They could have gone to another Outpost, crashed, left planet and returned later, of several other scenarios that align what we saw in Rey’s vision with what Kylo said. Also, due to the dark side there is no guarantee that what Kylo said is accurate, though all the other dark side users typically say the truth, but in ways that slant things there way so I wouldn’t count on what he said being false, just misleading.