There was a thread on this a few weeks ago and a link to a person on I think Reddit who was a barman on Oasis, and he confirmed this was correct - they are guaranteed a minimum but the tips make up that minimum first then everything thereafter was an extra, It was done on the amount of drinks punched into the till or something.
No server works just for tips as such- they are guaranteed a basic. - Thats if the person was to be believed of course.
It's important to remember that the roles are seen as very good jobs compared to their home countries, they earn in a few months what they can earn in a year back home