Ah, it just looked under-engineered. I thought the weakness with the idler would be its rather flimsy adjustable mount, which looks like it'd flex under stress. I thought it would need a triangular carrier made of plate - two mounts at the bottom - with a tensioner adjustment slot on one mount.
I guess such a small idler is going to rev so fast and present a high load to small bearings. A bigger one would allow a larger spindle with larger bearings and use of an off the shelf output shaft sprocket. But maybe there isn't the offset to allow all that.