Sounds like they'll finetune the model on your existing codebase. Even if Microsoft/Github offers this functionality, it's the kind of thing where it may be worth paying a premium to have good customer support telling you exactly how to make use of this.

It's like how there are plenty of people paying for DB software which is 'worse' than mainstream free ones, but they really like the fact they can call experts who will tell them what they are doing wrong with query optimization, etc.