The always insightful Aparna Mathur of the American Enterprise Institute explains why they’re wrong (my emphasis):

Despite the steady positive news in the monthly jobs report, a recent talking point, oft-repeated, is that too many people are holding two or three jobs in order to make ends meet. Rep. Tim Ryan brought this up during the recent Democratic debate arguing that “the economic system…now force(s) us to have two or three jobs just to get by.” In the earlier debate, Senator Kamala Harris suggested that the low unemployment rate is not cause for celebration since for many Americans, this still meant working “two or three jobs” in order to make ends meet. Similar sentiments were expressed by Rep. Alexandria Ocasio-Cortez (D-NY) (ick!), who during a PBS interview said “Unemployment is low because everyone has two jobs. Unemployment is low because people are working 60, 70, 80 hours a week and can barely feed their family.

these statements are inaccurate, both because they reflect a poor understanding of how the unemployment rate is measured, and because they exaggerate the phenomenon of multiple jobholding in the US.

…The Bureau of Labor Statistics (BLS) measures the unemployed by counting the number of people who are jobless, available for work, and have actively looked for a job in the prior 4 weeks. This means those working a part-time or full-time job are not counted as unemployed. The same person holding multiple jobs does not bring down the unemployment rate any more than that person holding one part-time or one full-time job does.