"Phonetic reductions," according to the article. Although in elementary school we learned to call them "contractions." (Isn't. I'll. 'Cause. Didn't. That'll. Who's.)
It's mumbling if the listener doesn't understand the meaning of the word or words at all. Which might be data compression, but I don't think of mumbles a being any more concise, just less intelligible.
> Although in elementary school we learned to call them
> "contractions.
This is the part of your understanding that is incorrect. You're talking about contractions, which linguists call Synaeresis [1]. The article talks about Vowel (Phonetic) Reductions [2], which are changes in the sounds that the speaker emits, not the complete removal of the sounds.
But the article has examples that are clearly different from contractions:
> ...the word fine would be pronounced less distinctly in a sentence like “You’re going to > be just fine” than “The last word in this sentence is fine.”
This is again, just semantics, but I suppose you can say it's something akin to "concatenating" or "slurring," although the latter has its own connotations.
It's mumbling if the listener doesn't understand the meaning of the word or words at all. Which might be data compression, but I don't think of mumbles a being any more concise, just less intelligible.