It's hidden in a subtab in the article, but it says you can get it with the free tier of chatGPT app for Android or iOS by tapping the headphone icon to the right of the text box.
This is an old message I only just noticed, but maybe that's better anyway....
...because my ChatGPT app is up to date and doesn't have a headphone icon to the right of the text box. There's a soundwave icon that lets me use speech to text, but that's not what this is.