I have not seen any instance of this frequently-made assertion which is at all justified. It seems to rely on a definition of "understand" which is more about spirituality than actual observable evidence (they clearly can comprehend even complex tasks well enough to execute on them, and if you won't call that "understanding", you're playing word games rather than stating an objective fact).
Likewise, agents can literally come to a greater understanding of a problem through trial and error, and there are plenty of mechanisms to retain that knowledge. If you don't want to call that "learning", you're just making a choice to define it in a way more restrictive than how we use it for humans, and intentionally making communication more difficult.
"Understanding" has enough philosophical leeway in its use to allow at least the possibility of sentience as a prerequisite.
This is where the discussion about LLM capabilities becomes genuinely difficult, and dismissing that difficulty as "word games" or "spirituality vs evidence" is not helpful.
In fact, I'd argue that statements about what "is" and "is not" sentient relies on even more spirituality and word games for anything that isn't a terran tetrapod.
For a meaningful -- "helpful" -- discussion on such things, one has to assume that everyone is choosing a definition which is closer to the median usage and relies on not being totally subjective. Furthermore, given the breadth of options, it should be assumed to be a definition which allows which permits the form of the question to be meaningful, rather than begging the question -- if your definition is tautological enough that non-biological entities can't have understanding, you're just expressing dogma rather than having a discussion.
Anything else is bad faith, or assuming bad faith on the part of the participants.
I do not think it is at all unreasonable or "fringe" to regard understanding as involving intentionality: ie a directedness of thought toward the object-relations being "grasped". That may not be the only possible conception of understanding but it is a mainstream philosophical idea.
In fact, I'd argue that statements about what "is" and "is not" sentient relies on even more spirituality and word games for anything that isn't a terran tetrapod.
Then you seem to be confusing "hard to understand" with "meaningless".
you're just expressing dogma rather than having a discussion.
Anything else is bad faith, or assuming bad faith on the part of the participants
Have a think about that (repeated) tone before responding.
Fwiw I am a long-time believer in consciousness being fully realisable in machines; I think the jury is still out on LLMs.