Edit: I think I hit a lot of nerve on some people but here is my thinking, a cancer acts like a single cell because the very first cell stops functioning with the benefit of the host it’s more into itself. Some cancer will live outside the body can be cut and still multiply ain’t that a behavior of a single cell? E.g. Henrietta Lacks another is CTVT.
You are incorrect. Tumors cells can coordinate to request blood vessels to be created for it.
“[tumor cells] send out signals called angiogenic factors. These encourage new blood vessels to grow into the tumour” Source
And the meer act of being a solid connected clump makes it multicellular. Because its literally made of multiple cells.
Your right by definition it is a multicellular, though for the sake of discussion. Some single cell let say a colony of fungi/yeast would look into the eyes of a person as a clump. In their environment they would need to communicate in order to survive. Won’t that what angiogenisis looks like in the overview of things, a cancer cells coordinating each other for the sake of survival using their sorrounding environment. In this case they send singals to the body to give them nutrition and the body does that by giving them vessels.