This is the 2nd similar report concerning the JXA version of this method. I suppose that there’s some glitch somewhere, @cgrunenberg would have to assess.
According to the scripting dictionary, your call looks ok.
I suggest that you use const whenever possible instead of let and var, though. That prevents involuntary modification of values and leaking of variables into the global scope (where they don’t belong).
Edit: Running the script in Script Editor shows these messages
app = Application("DEVONthink 3")
app.getRecordWithUuid("1626AE02-7B82-4404-8074-8683B71A69BE")
app.summarizeHighlightsOf({records:[], to:"markdown"})
So, obviously, the records list passed onto DT’s guts is empty. Which it shouldn’t be.