He wasn't worried about them getting OVER the barricade, as it was only a foot between the top of the table and the ceiling, and another table had been jammed up there to prevent crawling. What he was worried was that one of them would try grabbing the top and yanking. Hence why he was growing the table and walls together to prevent that, and melting the table legs into the tables to strengthen them. Eventually he would completely seal off the stairs from the upper floors, leaving them to have to use a window to get in and out, as his next trick was to use some metal and another table to add another layer behind the slowly merging one.