I'm sure about the 68cm measurement as I did that one twice.
And I just checked a bike fit drawing done for my CX bike a few years ago....and the seat tube all the way to the top of the saddle line was 696mm. So maybe it's close. Maybe my legs are stretchy...
In that case, even being conservative (you may want to drop the saddle a cm or two compared to the road bike) you'd still likely have room for a 150mm dropper even on the Small size, if I'm not screwing up the calculations
70cm does seem quite tall. Not outside the realm of possibilities if you have long legs, but still