if i use the entity on platform signal with a fluid tank on the platform it outputs a fluid tank entity signal, this is not the same as the fluid tank item signal available through the signal selector in combinators, meaning conditions won't work
example: https://images.minecrafter.uk/EIGWGUJ.PNG
fwiw, you can simplify your logic by not even bothering with the entity reporting. Some fluid isn't going to just appear on the platform. : )
To be clear: "Inventory on entity" is NOT dependent on "Entity on platform", they act independent of each other.
Note: Yes, internally the entities need to be detected to read their inventories/fluids, but you don't necessarily need them amalgamated and written to the output port.