The IRA/SEP/SIMPLE box was not checked because your wife's TSP is a qualified plan and provided by her employer.
The IRS taxes required minimum distributions at your income tax rate such as any other taxable income you receive.
For more information on RMD's, the following link from the IRS may be helpful:
https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-required-minimum-di...
