I was counting on the 10 to 20 Celsius degrees of extra cold resistance
seedlings have. BRBR

Seedlings are not likely to have any more cold resistance than their parents.
Actually, for harsh climates, the usual practice is to graft the desirable
cultivar onto a very hardy rootstock. This is done with a wide variety of fruit
trees and ornamental plants.

Apple trees prefer sexual reproduction.

Did they tell you this? I assure you the apple tree does not care whether it is
a seedling, cutting, or graft, as long as it has enough water, sun, nutrients,
& favorable temperatures. The only difference is that a seedling will take ten
or 20 years to bear fruit, while a cutting or graft will start bearing very
quickly.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)