It's not that I don't understand that it would make sense for a burglar to wipe the sill first. But I don't think it is at all unreasonable if they didn't.
All that is required is for there to be room for both of his hands on the sill as he hoists himself up. There is plenty of room for that. He then swings his legs in quickly. If he swung his legs in over the right hand side of the window they might clear that side of the window sill of glass but not the left hand side. Hoist up with both hands on the sill, swing legs up and in over the right hand side of the window. There is almost no glass on the right hand side of the sill and there was glass on the floor. It also drags glass into the room without knocking it outside. I see this as a reasonable break-in scenario. I think that is exactly what happened.
You don't think his legs might hit the wall? It seems impossible to me that this scenario was the one used. There's too little width to allow a swinging of the legs.
In fact if didn't want to he could simply jump inside without touching the sill with his feet at all.
Hence the use of the term "spiderman" way back when