Rolling more frequently on the different roads means the paths have agency. It doesn't make the encounters not illusionism. If the first roll is a traveling merchant caravan, the second is a dragon, and the third is a band of mischievous pixies, what does it matter for illusionism whether the the encounters are all on the same day on the fast road, vs subsequent days on the slow road? The same rolls happen either way. It's not meaningfully different from if the GM is choosing those encounters and placing them on whichever path the players choose.
No, in the OP, which is what I was discussing, there is no mention of making rolls in advance.
@Reynard later mentioned that that's how he runs things, and that if the PCs backtracked to the other road a different set of rolls would be made. And I agreed in my response that in that case, there's no illusionism. But that is not what was presented in the opening post, which is what I've been discussing.