Winter root growth is widely-observed amongst people who work with tree roots annually (bonsai hobbyists and professionals, commercial ornamental tree growers, especially propagation-adjacent roles) in milder temperate climates (eg: Pacific Northwest).
In my time studying bonsai (past 7-10y) in every winter (Jan-Mar) I've repotted (anywhere from partial to full bare rooting, partial to full root structure editing) many PNW-native deciduous trees (alders, bigleaf + vine maples, cottonwoods, etc) as well as non-PNW-native deciduous (birches, beeches, elms, maples, hazels, hornbeams, stewartia, bald cypress, cherries/plums, quinces, snowbells, etc). In somewhat-mild-and-milder climates there is always some root growth going on. Such winter root growth is much more aggressive in conifers, particularly pines, but also in spruces, cypress-family species (junipers).
Here's a diagram from a paper showing scots pine and rowan/ash adding either root or vegitative growth in various parts of the year:
Temperate trees collect or spend carbon in the warm parts of the year, i.e. between bud-break and mid-summer. In cooler parts of the year, they do various things: store it in the wood, move it around (redistribution + retranslocation), spend it (future-season buds + cambium + root expansion) or carefully avoid spending it (dormancy).
Even in these periods they're still collecting sunlight if they can, quite a few deciduous species can photosynthesize at least a little bit directly through their bark -- young twigs have much thinner bark even in trees that get very rough bark (eg: black cottonwood). And evergreens are collecting sunlight any time mild-or-warmer conditions are in play.
Trees are active in some shape or form any time they are able to be. If you live in USDA hardiness zone 7 or warmer and have trees/shrubs outside you can notice this more easily than in colder climates (where the grow/no-grow seasons are more sharply bounded). Roots are not the only thing expanding in winter. Take a picture every day of a branch on a deciduous or evergreen tree and you'll see bud expansion.
Yeah but they aren't making this clear and see people misinterpreting this not realizing that it is in temperate forests only. I don't see how any tree roots can be formed when frost line is feet below the surface.