I believe that section is both confused and confusing. Sysupgrade (at least of a somewhat recent OpenWrt version) resets the partition table only if the boot and rootfs partitions differ. If they are the same in the new version, their contents will be replaced but the partition table stays intact -- including any additional partitions.
And even if the partition table has been reset, additional partitions can be restored by re-inserting them into the partition table. That's why one should always take down the partition cornerstone data before sysupgrading.
I generally recommend additional partitions not to start completely flush to the rootfs partition, so there's some leeway for future expansion of the default partitions. This hasn't happened yet, in fact the rootfs size shrunk in one of the recent releases, but the default partitions' sizes should be regarded as somewhat volatile. Personally, I start additional partitions a gigabyte into the drive, it's a nice even number.