There are three real life families of felines, the ones who purr but cannot roar, the ones who roar but cannot purr, and cheatahs who do not have retractable claws.
As for house cats being unable to purr and vocalize, I have often heard purrs within vocalizations, much like one can hear laughter within speech when one cannot stop laughing while talking.
Settings says that Enkis are close to nature, it would make sense that they would be a bit freer with body language and non-speech sounds such as purring or roaring, growling, hissing, etc.