A Squared - Well, if the Captain takes over at 400 feet, there certainly does seem to be an implication that the approach was unstable or otherwise outside of parameters. But the Crew reported that the approach was stable, on speed, on track and on glide. Not sure what the missing bit of information is that makes those two facts fit together.
From the NTSB statement
Flaps on SWA 737-700 were set from 30 to 40 degrees about 56 seconds prior to touchdown at LGA.
SWA stabilized approach criteria requires that the aircraft be fully configured by 1000'. That includes the landing flap selection. At 56 seconds from touchdown, the aircraft would have been at approximately 650 feet.; Below the stabilized approach altitude and requiring a go around.
Perhaps this explains the alleged call for a go around ?