I would have to agree and disagree. Android on Kobo is just a hack, made by some fans. It's not optimized. And why would it be? It's something someone did for free, in their free time. Android on Onyx is part of the product, that means someone got paid to make android run as smoothly and take as little of resources as possible.
Furthermore I find the comparison to be very fair for one single reason. This page
https://onyx-boox.com/shop/onyx-boox...id4-bloetooth/ has nice comparison chart, when it shows off differences between T68 and Aura HD. Onyx claims their device lasts up to 8 weeks of use, just like Aura HD. If they can make this comparison then it means they feel it's fair.
So when comparing hacked android to non android device I would say the comparison is not fair. But when comparing pro android to non android device I would say it's fair. Especially when it's a promise made by device manufacturer.
Edit: actually it seems Aura HD has 3.7V, 1500mAh battery(based on wikipedia page, not sure if can be trusted), while the link above says Lynx T68 has 1700mAh Li-ION Polymer battery. So it's probably only up to the system.
Edit 2: the difference might also lie in the hardware itself. Aura uses processor developed for e-ink devices, while T68 uses general purpose processor for low energy devices. Dedicated solution might be better. Also who knows about other components used. Since the device can't go into real sleep mode I guess some of the components in T68 aren't best choice.