Hello @gerko,
The file structure of TorizonCore image is a bit different (since it uses OSTree) from that of our reference BSP images. On the BSP, the kernel binary and device tree(s) are part of the bootfs.tar.xz archive. So you simply need to replace them in the archive (just like in the article that you already found).
This is why we recommend building the image following this article. The advantage of this method is that you will get the image in Toradex Easy Installer format in the output directory - which you can deploy using multiple methods.
In your case since you need to add a custom kernel module, this section would be the most relevant to you. In summary, you will create a custom layer and a recipe to build your custom kernel module. After that, you will build the torizoncore image using the command bitbake torizon-core-docker.