Characteristics of 'true' tissue

Why are sponges not considered to have tissue level organization?

Quite a few places seem to say that sponges do not have ‘true tissue’ and only have specialized cells.

Unlike Protozoans, the Poriferans are multicellular. However, unlike higher metazoans, the cells that make up a sponge are not organized into tissues. Therefore, sponges lack true tissues and organs;

The Wikipedia page on sponges says –

Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs.

I have not been able to find what defines true tissue, so I would like to understand what characteristics a group of cells must have for it to be considered proper tissue.