It probably was FSD -- FSD is available to pretty much everybody in April, so it seems unlikely he was using Autopilot.
But what seems likely to me is that the driver had his foot on the accelerator. FSD is relatively timid. For example, it will slam on the brakes if it sees a pedestrian that it thinks might cross in front of the car. The driver can press on the accelerator to prevent this. I've gotten used to just keeping my foot on the accelerator for one particular road where FSD is particularly prone to slowing down for no good reason.
But what seems likely to me is that the driver had his foot on the accelerator. FSD is relatively timid. For example, it will slam on the brakes if it sees a pedestrian that it thinks might cross in front of the car. The driver can press on the accelerator to prevent this. I've gotten used to just keeping my foot on the accelerator for one particular road where FSD is particularly prone to slowing down for no good reason.