Is pumping the control column an acceptable technique during flare?
He’s not pumping the yoke per se;  rather the pilot is making a series of quick, precise corrections to the yoke in order to track a precise glideslope to an aimpoint while on short final.  This process can be exacerbated by gusty winds on final, poor throttle control, etc.

Here’s a better example from the cockpit of a C-2 Greyhound CODS aircraft making a Case 1 trap aboard Carl Vinson (CVN-70).  Note the hundreds of minute inputs the pilot makes to the yoke, rudder pedals and throttle on lineup, in close and at the ramp.  He’s applying pitch inputs to correct for deviations from glideslope, throttle inputs to correct for airspeed deviations from Vref+5 kts, aileron and rudder inputs to correct for deviations from lineup on the groove, and rudder inputs to correct for yawing brought on by changes in power.  It looks exhausting.