[chrome111]

They're referring to hiragana being highly phonetic, where each letter is a fully formed sound so practicing them can give a solid base for pronunciation.

Personally I think there might be some correlation vs causation on hiragana vs pronunciation, many people with poor pronunciation just have a mostly-English speaking social circle, and as such likely also don't feel a need to study hiragana. I feel just listening to Japanese can help get your brain used to the sounds and on the flip side my first recommendation to Japanese to get better at English conversation is to watch lots of US TV, hearing words over and over from many voices eventually adds up to a nice base for sounds. Still need to study for vocab though.