I have never heard of a rimfire barrel that is hardened to anywhere near the limit of what would be reasonable for the alloy employed.Stainless steel is softer than chrome moly so many competitors change barrels after about 15,000 rounds.
In other words, while it might be true that the hardest chrome-moly steels can be hardened to higher levels than the best stainless steels, it's purely academic since those figures run up in the high 50 to low 60 RC levels.
Rimfire barrels are 20-25RC at best. In other words, the hardness of a rimfire barrel has absolutely zero to do with what type of steel alloy is used and is purely a manufacturing decision.
If any manufacturer was willing to endure the pain (cost) of machining such hard steel, they could easily make a stainless barrel that registered 2x or 3x harder than the typical carbon steel/chrome-moly rimfire barrel.
I can't come up with a plausible reason why that might be true for rimfires, given that it's fairly common knowledge that centerfire stainless barrels seem to last longer than carbon steel/chrome-moly steel barrels. Not due to hardness differences, but because the stainless steel seems to be slightly more resistant to throat erosion....quality chrome-moly barrels can outlast several stainless barrels used for rimfire benchrest.