If the agent you are running really wanted to it could easily find a way to mount the windows folders and read them all. WSL isn't a security boundary, you are only barely more protected than people running grok in their home directory.
I never claimed WSL2 is a security sandbox. I am saying running it in a container or WSL2 allows you to severly limit the blast radius. I am not expecting the agent to be malicious, but I am expecting it to do unexpected things.