The Google search app named “Google” on iOS 13.7, iPhone X. Norway.
The button is there for me, and hum to search works.
Screenshot: https://snipboard.io/ITZN9A.jpg
When I first installed the app after reading the article, to test it out, I couldn’t find it even when I pressed the microphone symbol.
But after rereading the article I installed it again and when I went and tapped the microphone I saw the Search a song button.