The participants next spoke a certain sentence in their native language that was recorded. The American students, for example, were instructed to say, "When the sunlight strikes raindrops in the air, they act like a prism and form a rainbow." This sentence includes every sound in the English language.
The sentence very clearly and obviously does not include every sound in the English language. There is neither a long or short 'oo' sound, for instance; and while the letter y occurs, it doesn't make its quasi-consonantal y sound (as in the beginning of 'you' or 'year'. You won't find j as in 'jump' or g as in good. So lots of sounds in the English language are missing.
But it's often interesting to see why someone makes a mistake like this. The claim is not made in the actual paper. (As a side note, why don't science journalists actually provide links to the papers they are talking about? In this day and age there is no excuse for not linking to papers that are available to the public online. This tiny little change would massively improve online science reporting.)
As the paper notes, however, the sentence is the first sentence of the Rainbow Passage. The Rainbow Passage is a public domain text that is widely used for articulation drills, speech recognition testing, and language studies because the whole passage provides a very broad selection of English-language sounds. I doubt any linguist would insist that it includes every sound you ever hear in the English language, but it does include all the ones you are likely to hear in ordinary conversation. More importantly, it is what is called a 'phonetically balanced' or 'phonemically balanced' passage, in which the sounds, or phonemes, are found in proportions fairly similar to those in which they are found in ordinary conversation -- very common sounds are very common in the passage, less common sounds are less common in the passage. So the reporter, apparently trying to pin down why a particular sentence was used, seems to have confused the first sentence of the paragraph with the whole paragraph.
The Rainbow Passage is as follows:
When the sunlight strikes raindrops in the air, they act as a prism and form a rainbow. The rainbow is a division of white light into many beautiful colors. These take the shape of a long round arch, with its path high above, and its two ends apparently beyond the horizon. There is, according to legend, a boiling pot of gold at one end. People look, but no one ever finds it. When a man looks for something beyond his reach, his friends say he is looking for the pot of gold at the end of the rainbow. Throughout the centuries people have explained the rainbow in various ways. Some have accepted it as a miracle without physical explanation. To the Hebrews it was a token that there would be no more universal floods. The Greeks used to imagine that it was a sign from the gods to foretell war or heavy rain. The Norsemen considered the rainbow as a bridge over which the gods passed from earth to their home in the sky. Others have tried to explain the phenomenon physically. Aristotle thought that the rainbow was caused by reflection of the sun's rays by the rain. Since then physicists have found that it is not reflection, but refraction by the raindrops which causes the rainbows. Many complicated ideas about the rainbow have been formed. The difference in the rainbow depends considerably upon the size of the drops, and the width of the colored band increases as the size of the drops increases. The actual primary rainbow observed is said to be the effect of super-imposition of a number of bows. If the red of the second bow falls upon the green of the first, the result is to give a bow with an abnormally wide yellow band, since red and green light when mixed form yellow. This is a very common type of bow, one showing mainly red and yellow, with little or no green or blue.