I believe I read the center seat in the second row folds into the floor on the LE and XLE though, not completely sure though about this.
From the pics it just appears they can fold forward, but not into the floor. I’m no engineer, but I think the batteries might be in the way. All the 2021 Siennas are Hybrid , even the spare tire is an option now on all new Siennas. The Center seat on row two is just like the older models, it’s removable and can be stowed in the rear. Another thing, the center console is much larger to accommodate a fridge and a vacuum system so now it extends into the cabin of the van with the 2nd row, taking of more space and making getting in and out of the van a little more clumsy. In the 2020 and back vans some have a center console that can extend to allow the cup holders to be closer to passengers in the 2nd row, but you can retract that part of the console to allow easy entry and exit from the van, now the console is a fixed item. I prefer the vacuum in the back of the van like in mine, now it’s in the center, I mean it’s not like you can use it when you're driving.