I don't think namepsace CAP_SYS_ADMIM grants you access to write non namespaces sysctls like modprobe_path

You're probably right, but that seems like the less important part of this. At that point you've already got an out-of-bounds write. Another comment speculated that you could use PageJack as an alternative exploit path once you have that primitive: https://news.ycombinator.com/item?id=48069623