Does tiling on a second level subfloor with cement already require uncoupling membrane?

Hello,

I am currently working on remodeling a second level bathroom. Photos for reference: https://imgur.com/a/b3uc0AT

When I did our downstairs bathroom, I used an uncoupling membrane on the concrete foundation floor, which I thought was due to it being the foundation and movement and all that.

However, turns out the upstairs bath already has concrete over the subfloor (which is plywood I believe, not OSB). The goofy thing is it looks to be poured? It would've been a nightmare to demo and after being cleaned up looked to be in decent condition, so I patched the area where the vanity was with Durock. We will be doing an open bottom/leg vanity so that is the reasoning behind that.

In any case I just finished painting and am ready to start the floor tile. Problem is I keep finding mixed or uncertain answers on whether I need to use an uncoupling membrane since I already have concrete over subfloor. This in theory acts as an uncoupling membrane correct?

One tiler told me you don't double up so it's either Subfloor>concrete/back board>tile or Subfloor>membrane>tile (so never subfloor>concrete>membrane>tile). Is this correct? Since its not on the lower level foundation am I safe without one?

Thanks!