The IRA/SEP/SIMPLE box is only to be used for distributions from IRAs. A 401(k) is not an IRA. (SEP-IRAs and SIMPLE IRAs are types of traditional IRAs.)
See the instructions for Form 1099-R: https://www.irs.gov/pub/irs-pdf/i1099r_21.pdf
Only an inexperienced plan administrator would make such a mistake.
