This is a fairly simple one to disprove. The popes coffin has a very specific shape, it's wider at the head and tapered towards the tail end. In both the image this claim has linked and a multitude of other images you can see that the cross is oriented with its top towards the head of the coffin and its bottom towards the tail. On top of that you can even verify the position of the pope in said coffin in many images before it was sealed. The perspective of this photo is from that of the cardinals honoring him they stand at the popes head so in their perspective the cross would be inverted, but the simple solution to that is standing in front of the coffin rather then behind.
(A different angle of the image linked)

(An image of the coffin with the former pope oriented in the manner described above)