Nobody can predict the specific future of any plant but theoretically if your plant was healthy, it should start to grow new foliage soon. I don't really understand the advice you were given. Why would anyone want to look at a plant trimmed down to the soil at any point in time? In the future, if you make much more minor adjustments to your plant much more often, that should never be necessary.

I think the assumption behind the advice was that only some of the stems would be woody, not all of them. I'll keep the soil moderately moist and hope for the best. Maybe even throw it over a heat mat.

It should bounce back just like Tiffany said. Fresh mix should help as well. Outdoors it can really thrive but indoors should be fine too. My first Medora plants were nearly 40 years ago and grew like weeds on our small deck at our apartment. Back then it was sold as Trout Leaf Begonia. It was a quirky thing - bolting up to about 2 to 3 feet tall (maybe taller) with lots of bushy growth and then languish during winter months. Lost them after a few years, buy more as I found them.

I typically don't prune Medora that much unless I wanted to start new plants or it was too tall to put under lights in winter. Other canes I prune to the ground in recent years and they do bounce back better than before.

I'm not sure a heating mat would be appropriate, begonias are typically native to cool mountain sides, cave entrances, etc.

I doubt you would need a heating mat unless you are in near freezing conditions but then that only helps the roots.