It's a "source" that cannot be reproduced or actually referenced in any way.
And all the other examples will have a chain of "upstream" references, data and discussion.
I suppose you can use those same phrases to reference things without that, random "summaries" without references or research, "expert opinion" from someone without any experience in that sector, opinion pieces from similarly reputation-less people etc. but I'd say they're equally worthless as references as "According to GPT...", and should be treated similarly.
And all the other examples will have a chain of "upstream" references, data and discussion.
I suppose you can use those same phrases to reference things without that, random "summaries" without references or research, "expert opinion" from someone without any experience in that sector, opinion pieces from similarly reputation-less people etc. but I'd say they're equally worthless as references as "According to GPT...", and should be treated similarly.