A collection of cells can maintain their individuality, and have almost nothing in common but their location. No doubt all multicellular organisms today are descended from such collections.
To start doing anything useful for survival, the cells of such a collection can begin to specialize: some for locomotion, some for digestion, some for protection, and so on. Specialized cells group into tissues, and then into organs.
Organisms with cells responsive to electrical stimuli, which occur only in the animalia, can develop specialized tissue for movement, leading to muscles. They can also develop sensory organs, because excitable tissue gives them a mechanism for transmitting signals, and acting in response to sensory inputs.
A collection of cells without any of these specializations may develop into a syncytium, a mass in which there are many cells sharing a common cytoplasm, with many individual cell nuclei lying in it. This arrangement allows them to share nutrients without having any specialized tissues or organs for doing so. Although there are multiple nuclei, it can almost be said that they live in a single, very large cell. Not having specialized tissues, they don't display the characteristics of multicellular organisms, and not having excitable tissue, they don't develop nervous systems.
This is the strategy some sponges have adopted, and that's why such sponges are not regarded as multicellular organisms.
Not all sponges are quite as simple as this, and have some differentiation in cell function. Whether these form tissues or not is a matter of definition.
So some sponges are almost entirely syncytial, and can't be regarded as multicellular, although they are metazoan. Some are a little more complex. And that explains why sponges will sometimes be regarded as multicellular, sometimes not. I guess it depends on the sponge.