Native speakers perceive sentences of their own language as sequences of separate words.
Conclusion
This perception is an illusion.
rephrase: Native speakers are wrong to hear sentences of their own language as sequences of separate words (or, )
Evidence (this is shown by the fact)
Travelers who don't know a local language hear an unintelligible, uninterrupted stream of sound, not sentences with distinct words.
Evaluate
This argument boils down to this structure.
People X say Thing A. But they're wrong. This is shown by the fact that People Y say Thing A isn't true.
We sometimes nickname this a He Said / She Said argument, meaning that it's one person's word / point of view vs. another's. The author often arbitrarily sides with one view over the other without justifying their pick (or their attempt to justify favoring one side over the other is super sketchy).
When I listen to Spanish speaking radio, I have a very hard time separating the stream of sound into distinct words. Meanwhile, I'm guessing a Spanish-speaking listener does can comfortably hear the distinct words in a sentence.
So which of us has the "correct" perception?
We could say that my perception is more accurate, since I have fresh ears and am not biased by familiarity. We could say that their perception is more accurate, because they have the necessary faculty (they know Spanish) to accurately judge whether sentences are comprised of distinct words.
Goal
Look for an answer establishing a Linking idea like,
Or look for an answer stating the author's assumed preference in this He Said / She Said debate:
Or look for an answer ruling out an objection: .