The statement, "Adobe RGB squeezes colors into a smaller range (makes them duller) before recording them to your file." is not wrong if you interpret it as Adobe RGB has a wider range of colors but are represented only by a RGB triplets, each in the range of 0-255.
In both sRGB and aRGB, each color is represented by a triplet of R,G,B, each having a value of 0-255. Hence, the number of colors represented by the triplet in sRGB and aRGB are equal to 256x256x256. However, the actual lightest green in aRGB can be lighter than the lightest green in sRGB. Similarly for the darker green in aRGB to be darker than the darkest in sRGB. Thus, aRGB is trying to squeeze a bigger range of greens into the same 256 values that the G component in a pixel can represent. As a result, the tonal change from say a green value of 200 to 201 will be show a bigger jump visually in a aRGB colorspace than in a sRGB colorspace. With the bigger tonal changes for each discrete value change, aRGB will appear less smooth than sRGB for portions of image that has continuous tonal changes. Hence, it may appear duller.
For more understanding, you may want to read the article in
Cambridge-in-color.