Was Cuneiform Originally Sumerian? Linguistic doubts about the world's oldest writing

From a channel Lumen includes in full: NativLang animates the linguistics of the world's languages and writing systems, from cuneiform to Oceanic phylogeny, with careful sourcing and the creator's decade-plus of linguistics study.

A 18-minute video from NativLang's catalogue; the title names its topic.

Video by NativLang, played through YouTube's embedded player.