Google does not permit any kind of use of Product Sans for any third parties.
Here is the license of the font:
Google offers many fonts under open source licenses. This is not one of them. Please see fonts.google.com for options you can use.
As you can see, it doesn't explicitly grant any permission to use the font. When thinking with a strictly legal mindset, you can't just assume that you're allowed to use the font simply because the license doesn't prohibit it, or because “others use it too”.
Here's a statement from a Google employee who works in the Material Design team:
Unfortunately Google Sans is not a font we license to developers. It's based off our logo and is used to express our branding in particular.
Furthermore, I sent an email to Google's design team about the license. Their reply is very concise and should not leave anything to debate about.
So, to rephrase:
Product Sans and Google Sans are not licensed to third parties for any kind of use.
It would seem that Scott's answer is not correct, and the font was uploaded to befonts.com without permission and with a wrong license, by someone who used “google” as their username.