What does having invented a term have to do with this?
The Otis company invented the term escalator, and even had a trademark on it for a while, but does it mean that you'd only call one an escalator if it was made by them?
That's literally what the trademark means. At some point things become so dominant and generic a trademark is no longer successfully enforceable and you get escalators, bandaids, linoleum, taser, gasoline, etc.
“Transformer” is the name of the algorithm behind popular LLMs.
GPT is the name that openai gave to their models early on.