401k, non-US citizen, non-permanent resident in Japan

I've been reading through the posts about 401k taxation but haven't found information about my personal case, and I'm confused by the tax treaty.

I'm European, I worked in the US for a couple of years and still have a 401k from my employer.
I have no plan to go back to the US and I've been struggling with managing the account as I don't have a US phone number anymore, so I'm considering closing it even if it means 10% penalty. I don't plan to remit the money to Japan, I want to move it to my home country.

I have been living in Japan now for less than 2 years so I'm still a non-permanent resident tax-wise. I've read that as a non-US resident, there would be a 30% federal tax withholding. Is that 30% tax unavoidable in my case (non- US citizen) or should I file a W-8BEN and get it taxed in Japan? If I don't remit anything to Japan, will it even get taxed? Sounds too good to be true, and I don't want to cheat the system.

Thanks!