Not too sure about it being "confirmed random" the only random part of it is which ninja you get. Every 60 scrolls youll get a "rare" every 400 youll get a "super rare". Rare and super rare counters are interdependent, as well as the counters for different treasures.

Ex: Sage Treasure 48/60 and 248/400 Tendo 12/60 312/400 Sage Treasure 50/60 350/400

Now this is of course the majority of 10x pulls with free draws added in. Their may be a range within the last 10 for it to be random. Ex: Sage Treasure 50/60 Single pulls 51-60 may have a equal likelihood to draw a rare.

I've been able to "call it" on every rare and super rare drop since I've started. If they claim it to be RNG based they probably have a built in function to guarantee a ninja after a certain time period for unlucky people (like Diablo 3 did in one of their later patches to guarantee a legendary drop after I think 2 hours of playing if you havent already found one)

TL;DR - My luck is * and I get a rare every 60 pulls, a super rare every 400.