I think Feijoas can withstand far more cold and frost than most people suspect, although it might as well depend on the soil, frost duration and microclimate.
I can only speak from my own experience: I have one Feijoa plant grown from seed permanently planted outside in the ground since 2017.
The frost period here usually extends from November to February and March (around 75 frosty days on average).
Winter 17/18 was exceptionally cold with minimum temperatures of -18°C / -0,4°F (see image for detailed temperature chart). During these cold spells I only covered my Feijoa with some fleece and some conifer twigs and no active heating or whatsoever. To my surprise my plant survived almost undamaged with only some frost burnt leaves on top and has recovered perfectly fine since then.