If they are each under the filing threshold** for filing MFS, then they can file MFJ, because they will both get all their withholding back, either way. It's just the convenience of filing one return; the result is the same either way.
But if one spouse has (for example) $12,000 income and the other only $2000. The one with $12K income would not get all his withholding refunded by filing MFS . So in that case they would not qualify for the exception. They may want to file MFJ to take advantage of the larger standard deduction and get all his withholding back.
**Filing threshold (typically $6300 in 2016-17; $12,000 in 2018) )
