Setting aside their interpretation of the results, which is a bit heatedly contested (and I believe they were focused more on syntax rather than speech), there

Setting aside their interpretation of the results, which is a bit heatedly contested (and I believe they were focused more on syntax rather than speech), there are really two questions to ask that I think are relevant to the discussion: 1) Is it POSSIBLE for the speech of an adult language learner to be indistinguishable from a native speaker? 2) If (1) is true, are such instances common? The reason (1) is important to ask is because a strict critical period hypothesis may rule this out by saying once the brain has matured beyond a certain point (age ~12, for example, or possibly younger for phonology), you can no longer acquire a language to the extent of a native speaker. But there is plenty of evidence that (1) is true. In the Hyltenstam and Abrahamsson literature, they are able to find quite a few learners whose speech is indistinguishable from that of native speakers. They then run a battery of more strict tests (syntax/knowledge tests IIRC) against these learners that reveal slight differences. However, (2) is certainly false. It's somewhat rare to find an adult learner who truly sounds like a native speaker. That, however, isn't evidence in favor of a strict critical period/maturational effects hypothesis, since there can be many reasons for failure. My intuition is that there is much less support for a strict critical period hypothesis these days. Most people now talk about a 'sensitive period', which reflects that there are a broad range of factors that favor successful acquisition in children but that adults have decreased sensitivity to.