Wish this is in English :-(

8pc 18650 3.5Ah Sanyo GA cells = 103Wh and 260w continuous power, 390g.

Panasonic has a 3.9Ah cell but the it can't hold the amp draw.

I'm not sure they find much better than 18650, check energy density :

http://eu.nkon.nl/sk/k/ncr18650ga.pdf > 228Wh/kg at 2.8C

http://kokam.com/wp-content/uploads/2016/03/SLPB-Cell-Brochure.pdf > max 260Wh/kg at 0.5C, wasting already 4% as heat

They may even use primary cells as they do not need to use them again. But I'm pretty sure most of those guys have to get what's available on the market except the top teams. Even with of the shelf tech its easy to do and to hide. Most of the shelf cells can accept up to 25% overcharge safely for a few cycles.

Few years ago already smart guys could DIY 7.6kg mtb with 240Wh (not hidden) https://www.electricbike.com/lightest-bike/

it was meant to happen sooner or later, everything is valid to win in this game.