Motorbike kit is surprisingly similar in general design to bicycle kit, but is generally more stoutly build.
A motorbike pannier will be used at speeds of 50-100 km/h and has to survive the driving rain at that speed, compared to a bike that might do 20-30 km/h
The downside of this robustness is that motorbike parts are heavier and tend to be larger. The size might work for carrying larger loads, but the underlying challenge is the weight.
A motorbike has anything from 3 to 300 horsepower (2,000W to 220 kW) where a cyclist is well under 1 horsepower (750W) so the extra mass is less important on a motorbike.
I suggest that if you can back out of the purchase without penalty, do so.
If you're stuck with the bags, look at ways to adapt them to your bicycle. Ideally in a way that is reversible, so that a future motorbike owner might undo some stitching and get them back to original. I'd suggest not cutting anything if you don't absolutely have to, and if you take off any fittings, then store them for future reuse.
Its possible you will have to extend your rear rack, or adjust hooks. Motorbikes do not suffer from "heel strike" in the same way it affects cyclists.
Based on photos you may be okay. That rack is definitely for a bicycle so should hold up to 20-25 kg. Your bike frame should have a couple of holes down by the rear axle for the struts. You will ideally have a pair of threaded holes in the seat stays to connect the front of the rack - some bikes don't have these.
The velcro straps can rest on top of the rack but there's no "clamp" to hold them down or to stop the whole bag-set from sliding off the back. I don't see any hooks on the rear of the bags to clip on top of the rack's side rail.
Cable ties can help, but they're generally not rated for holding a lot of weight, and can suffer from UV degradation. Even the black outdoor ones will fail in time.
Those bags are not small either - You'll definitely feel them in a headwind.
If it were me I'd still try and back out of the sale as unsuitable first. If you're stuck with them, then it may be possible to make them work by modifying them, and possibly extending the rack so it goes further backward, though this can make the bike feel weird with too much rear-weight.