Different linux distros can put things in different places. So the fact that the extlinux.conf path is different doesn’t matter. The SiFive instructions are correct for the OpenEmbedded images that they produce. The Ubuntu instructions are correct for Ubuntu.
You can’t boot without the SDcard. The board will run u-boot from the SDcard, and then u-boot can optionally use either an SDcard root or a NVMe root depending on the extlinux.conf contents. In theory, the only thing you need on the SDcard is the u-boot partition if you have an NVMe root.