The main difference is that FFXV sort of falls into this storytelling style by accident, not because they purposefully wrote it that way. There's a ton more dialogue in FFXV than any of the Souls (or King's Field, etc etc) games, and it tries to present a forward-facing story all the time. Clues about the meat of the story in Souls is linked to supplemental bits of writing hidden around the world in relevant (usually, anyway) places. There's a logic to it, when it's at its best at least.
FFXV is very up front about its story beats and dedicates a lot of time to direct exposition, it's just not very good exposition. There is supplemental writing which is sort of like Souls, I guess, but the supplemental writing in FFXV is 1. in a movie not included with the game and 2. in radio shows (I actually really liked these, but I wish they played in the car instead of forcing you to stand still in restaurants) and books that are often in places completely irrelevant to the main story.
Basically, any similarities between the two seem accidental to me. FFXV is way too direct with cutscenes and dialogue to make me believe they were trying to go for the style of mystery the Souls games do. I'm not even saying one style is better than the other, I just fundamentally don't believe that's what they were trying to do with FF.