I’m not sure about that if they are laid down and heavily mulched, or is the zone rating because of days to maturity?
I have Suffolk Seedless, Somerset Seedless, and Vanessa that have all fruited this year. I have them in an unheated greenhouse, and it gets just as cold under that plastic as outside so this winter it was -38C. The greenhouse is to extend the season so I can get them to ripen. The mulch was very thick though, about 2 feet deep.