As the campaign doesn't start until the day after tomorrow, you may have to be patient before you can see exactly which bikes are involved. I suspect it will simply be the ones from
https://m-way.ch/sale/

At the moment the given example is
https://m-way.ch/e-bikes/e-citybike/sinus-tria-10-26-schwarz-matt/261.12.100416
is not yet on sale.

I would hurry then, because it concerns bikes in the sale. Not that you'll be disappointed like many others back in 2013:
https://www.srf.ch/sendungen/kassensturz-espresso/aergerliche-e-bike-aktion-von-m-way

Meanwhile, the month of September has begun and the three chosen bikes are known :
https://m-way.ch/cumulus

Well, the advantage is not that spectacular, because even without the Cumulus advantage you get a 20% discount : 0,8 x Fr. 3'199 = Fr. 2'559.20
https://m-way.ch/e-bikes/e-citybike/sinus-tria-10-26-schwarz-matt/261.12.100416?utm_campaign=lead&utm_medium=referral&utm_source=community.migros.ch

The Cumulus advantage applies to the full price and not to the sale price. Cumulus customers therefore save 10% compared to buyers without a Cumulus card.

It says :"Note: The offer cannot be combined with other promotions or discounts."

For almost the same price as the sale price of this e-bike, for example, there is a non-Cumulus e-bike with a 35% discount in the normal sale:
https://m-way.ch/e-bikes/e-mountainbike-hardtail/atv7/720-500-wh-navy/371.12.100019?number=371.12.100019

Fr. 2'599 instead of Fr. 3'999

Or a city-style e-bike like the sample is available with a 30% discount in the normal sale without the Cumulus advantage
https://m-way.ch/e-bikes/e-citybike/mavaro-active-city-race-red-w/jet-black/155.12.100489
for Fr. 2,414 instead of Fr. 3,449 .

It's definitely worth taking a good look and comparing.