The email can say whatever, it's just words :D, nobody forcing them to tell the truth - not to mention this is a spam automated email!
In the end, you will end up including some "ads.js" script into your codebase that does something it shouldn't do.
Their domain was registered only in December, so this is not some "established business".
Monetizing extensions is not easy and they know that extension developers are poor and frustrated so they offer them "easy solutions".