The warmth from the foundation could certainly have helped the underground portions (corm) to survive, but it does not seem possible that the above-ground portions would have survived the most recent winter, even if the house lacks insulation and the plants were leaning against it while the heat was on full blast.
Now we have had a number of winters that were mild enough for a banana to have its p-stem survive in the last few decades, but we’ve had deep multi-day hard freezes each of the last three winters (in my yard that meant winter lows of 16°F in Dec 2021, 17°F in Dec 2022, and 15°F this January). Are you absolutely sure the pseudostems didn’t regrow anew after each of these winters? You’re making an extraordinary claim, so I hope you’ll forgive my skepticism.
Looking at this chart I made a few years ago, some years where a banana might have survived above ground in a protected location (like against a heated building) include:
- 1991 (26°F low)
- 1999 (30°F)
- 2000 (27°F)
- 2001 (26°F)
- 2002 (25°F)
- 2003 (25°F)
- 2012 (26°F)
- 2015 (25°F)
- 2020 (28°F)
But those years are the exception (albeit the type of exception I keep hoping we’ll get again soon to help my avocados size up for once). The other years since 1990 have all generally been below 25°F, a threshold where banana p-stems start croaking even in sheltered locations without active protection (wrapping with insulation and string lights).