The problem is that tying progress to RNG means that some people will get done earlier and some people get done never (regardless of how many times they roll the dice). Statistically it's unlikely, but you could get all of your Demiatma on day one or roll the dice ad nauseam and never finish anyway. I don't know what statistics they're using or their target projections for how long it'll take people to get finished, but assuming the reported approximate 20% / 5% drop rate from CEs / FATEs (respectively) you need 80 CEs and 320 FATEs, on average, to get all 16 Demiatma from either exclusively.
On average. Like I said some people could get super lucky and get them all on their first try, and others can get super unlucky and never finish no matter how long they beat their head against the wall. The people saying "Well I finished it already so the drop rates must be pretty generous / fair." are just the ones who lucked out and / or did not get burned by RNG.