How does she even do that?
Practice, practice, practice. And a substantial gift to start with.
La Bartoli has the most amazing rapid-fire diction. Other singers fuzz the consonants in an effort to make the music sound "prettier" or get all the words in. Not her. Every note, every consonant is front and center.
Classical singing (here, oratorio) is extremely physical work, not made easier by wearing period costume. Even singers who carry excess baggage have rock-hard abs underneath.