LLMs do sometimes hallucinate even when giving summaries. I.e. they put things in the summaries that were not in the source material. Bing did this often the last time I tried it. In my experience, LLMs seem to do very poorly when their context is large (e.g. when "reading" large or multiple articles). With ChatGPT, it's output seems more likely to be factually correct when it just generates "facts" from it's model instead of "browsing" and adding articles to its context.