I think the claim being made is that the pause between consecutive syllables is (on average) the same whether they're part of the same word or not, so inter-syl

I think the claim being made is that the pause between consecutive syllables is (on average) the same whether they're part of the same word or not, so inter-syllable time is not a reliable indicator of word boundaries. Anecdotally, I think this is probably true of my speech most of the time (i.e. any time I'm not consciously enunciating). It might be less true for people with accents that have a slower cadence.