More of a risk of the seat hitting you if you leave it while its still going up I would think (physics not my strongest suit).
I think this is more a case of the pilot/WSO still having the upward vector the seat gave them so they lose none of the 'up' energy and it's about getting the parachute deployed asap at very low level like that. Separation wont occur until after the rocket motor on seat has burnt out so there's nothing to lose in ditching the furniture asap after that