Advanced is assumed knowledge for the Extension 2 course.
In MX1, you may encounter questions that explicitly ask for Adv knowledge instead. Rather, it'll test Adv knowledge, just at a harder difficulty. In MX2, you have a similar thing with MX1. But in MX2, you generally will not be asked questions that are specifically "harder Adv" content. Instead, the entire Adv course is assumed. For the MX2 paper, you're expected to have learned the Adv content, and just know it (without any explicit questions on it).
(An easy example of where Adv becoming "assumed knowledge" takes into effect is basically what you have hinted. Any MX2 problem involving geometric series is really just assuming that you know Adv off by heart, and are prepared to use it. What's less likely to happen is that you get a question which is explicitly just "harder geometric series". What's more likely is that you're asked a question that focuses on some other topic, but it just so happens that geometric series is involved somewhere within in the solution.)
Note, however, that you would need to understand Adv content anyway. As an MX2 student, you would still need to sit the MX1 exam nevertheless. In the MX1 content, "harder Adv" content is certainly examinable.