Deep work is a practice, a deliberate choice to focus without distraction on a demanding task. Flow is a psychological state where you are so absorbed that effort stops feeling like effort and time distorts.
They overlap but are not the same. You can do valuable deep work without ever hitting flow, and flow can appear in play or sport that is not work at all. In practice, the conditions that support deep work, a clear goal, no interruptions and the right level of challenge, are also the conditions most likely to trigger flow.