Well it isn’t if you read all of the details. However at face value Monzo very much market this as a “credit card” so I think some people won’t read past that. Which is why we get the surprised posts like this.
Personal opinion it’s a BNPL product and should be marketed like this.
Also, OP, if you’re using Flex day to day, which is a valid use of a credit card you’re better off with a real credit card which is rewards based.