I'd assume they will just bot your channel and would NOT go for what they offer.. If they honestly were an awesome marketing agency, then they have far bigger issues to worry about if that's their way of marketing their marketing services lol.
There are a lot of folks out there who will add a bunch of "fake subscribers" to YouTube channels for a price. However, it's against YouTube's ToS to inflate the subscriber numbers with bots, and YouTube has a way of removing fake subscriptions making it a waste of money to invest in buying subs...
The only way to "buy subscribers" legitimately is to use stuff like Google's paid advertising services.