I would guess that UpSampling would refer to an nn.Upsample layer, but also a transposed conv could be used. However, I think the former might be more likely.
Do you see any mentions on the number of parameters? If the authors claim the UpSampling layers don’t use any trainable parameters, it should be an interpolation layer (i.e. nn.Upsample), on the other hand if these layers use parameters it would point towards a transposed conv.