You didn't say what entirely makes up the $21K of AGI but will assume since you used "AGI" and have input the data, that the $21K includes only the taxable portion of their social security.
As far as the Sch D/8949 activity, the determination of gross income for the filing requirement includes capital gains but not capital losses, so your client may still have a filing requirement if they have losses offsetting the gains in your calculation AGI. You must use only the gains in the filing requirement calculation.
There is a whole list of items in Chart C of the 1040 instructions that lists other situations that would require filing. Probably the most common for older retired folks are if they had any distributions from an HSA, or Archer or Medicare MSA, even if those were used for qualified medical bills that has zero taxable effect.
Obviously this doesn't consider the state requirements either, so you'll have to check that also.