There's a difference between someone who genuinely believes what they are evangelizing, and paying marketing people to spread some corporate opinion.
The OP is referring to the second type I think.
Of course the two are not mutually exclusive, you could have an evangelist job for an open source company selling data protection software/services for example.
As far as I can tell I have no "buy things" motivation. I am not trying to get my sister in law to hire me to install and maintain an asterisk phone system or an instance of nextcloud or add a custom feature to vlc etc. Nor do I do those things in general where I would benefit indirectly if more people wanted those services generally.