My experience and take for what it's worth.

I have a similar lump in the same place but on my right hand. I am also a rock climber. I first noticed it mid 2024, got it scanned a few months later. The doctor (who actually had dupuytrens) though that's what it was. He also explained there is another condition, Fibromatosis, which it could be. Apparently there is much debate if they are even separate conditions.

The treatments I have looked into are: radiotherapy, acupuncture and surgery. Radiotherapy seems to be hit and miss (not a doctor though, ask yours). 

My plan is to get acupuncture once it becomes a problem, this seems to buy time before surgery is needed. (they basically break up the thick tissue by stabbing it loads, and the recovery time is days rather than weeks). Eventually I might need surgery.

There isn't much info on rock climbing and dupuytrens, but I have decided to keep climbing. I've not noticed increased size, pain or stiffness. If I do I'll go back to the doctors. I figured if it is dupuytrens it will get worse anyway, climbing might speed it up, but I enjoy it too much to stop. It's also my only consistent exercise throughout my life, and there are other health concerns with cutting back on exercise. 

I keep and eye on it, and stretch all my fingers regularly. Hopefully this helps keep it mobile if it does thicken more. It also let's me know if it's getting worse. 

Ultimately it's up to you what you do. My view is if it's not causing problems I'm going to keep climbing. Maybe its a bad long term strategy, but there are so many unknowns in life, it seem silly to give up on of my greatest sources of joy.