A non-US citizen residing in their home country may be considered a nonresident alien by the United States IRS.
See link to make sure you qualify:
A nonresident alien's income that is subject to U.S. income tax must generally be divided into two categories:
- Income that is Effectively Connected with a trade or business in the United States
- U.S. source income that is Fixed, Determinable, Annual, or Periodical (FDAP)
401k withdrawals would be considered FDAP:
FDAP income generally consists of passive investment income; however, in theory, it could consist of almost any sort of income. FDAP income is taxed at a flat 30 percent (or lower treaty rate, if qualify) and no deductions are allowed against such income.
The IRS only appears to be interested in United States holdings for taxation purposes.