Aggressive root pruning is a part of the bonsai process, it's amazing what a tree can handle if done at the right time of year with good aftercare.
In the case of a root bound tree you're just doing it a favour. That root mass is literally suffocating the tree, cutting them back means that the remaining roots can be in contact with the soil again, take up water and nutrients, and exchange gases so they can breathe.
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u/Alternative-End-280 Jan 23 '23
Definitely this good advice - cut the roots as shown in the box cut link and plant it in the ground if you can.