Literally survivor bias. The old growth forests that survived loggers were the ones they couldn’t get to for a profit. Those will be in or near geographic barriers like swamps or rocky terrain.
I've rarely hiked in harvested and replanted forests; these are boring. I did most of my hiking in central/northern BC and Alberta, Canada. These are some of the largest tracts of old-growth timber in the world.
The many old-growth forests I've hiked (sample: >1,000km, mostly dominated by conifers), with last burn times ranging from decades to centuries, have rarely boasted deep soil levels. This hypothesis is supported by existing research: https://www.researchgate.net/publication/225314103_Soil_Carb...
Old-growth coniferous forests are great -- but they're not a big "carbon store".
If you want land-use based carbon storage, I'd recommend investigating where the continent-wide, massive, deep, healthy carbon-rich store of topsoil and humous came from -- grazed grasslands and scrub brush.
Oh, North Canada? That's some tricky terrain. Bedrock is very near the surface, and you're right, it's hard to build up. Short growing season, for one.
The article you link mentions softwood forests being problematic for carbon accumulation. Not hardwood forests. Old growth temperate hardwood forests are quite rare now, and some of the apex tree species in those forests have very serious pathogens keeping them from re-establishing, more's the pity.
