I think they just wanted to have the selling power of the 700 battery exclusive to their Amflow bike, and decided Atherton were prestigious enough and low volume enough to also get one to accommodate their unique design.
Seems unlikely to me that bigger batteries will come out this year as they just released the 2 RS versions bringing their battery range to 5 already. But I guess Avinox is operating at a very different pace than the other brands, so who knows.
I think it's because there are more people that slot in to the 'I want the 600wh or the 800wh battery' than the smaller group that would only take the 700wh option. Keep in mind that the 700wh has to have a specific downtube design that only fits that battery. Also, it might not fit the upcoming 900+ wh option.
Regarding the extra power, the other batteries can already do the 1500 watts for 30 seconds on boost, so really the 1500 continuous is going to be a really small group that find that they benefit from it.
From my perspective as a new Orbea Wild (Bosch) owner, well I don't think the downtube will be any sleeker on the Avinox version which I'm sort of glad for because of the choice to offer the 600/ 800wh batteries'.
The Bottom Bracket area will look sleeker, but likely will (again!) not have appropriate chain ring protection like most Avinox bikes.