The offer is clearly more like a founder in that you are agreeing to a significantly below market salary in exchange for a pretty sizable chunk of equity. So whether the "title" of co-founder is given or not, they are treating it more like a minor partner/founder.
Given that this offer is equity heavy - you should understand things like vesting schedule, and dilution when additional investment comes in.
Given that this offer is equity heavy - you should understand things like vesting schedule, and dilution when additional investment comes in.