What they’re suggesting is to back up the whole disk, rather than any single partition. Anything you do to the partition to try and recover it has the potential to make a rescuable situation hopeless. If you have a copy of the exact state of every single bit on the drive, then you can try and fix it safe in the knowledge that you can always get back to exactly where you are now if you make it worse
Also, it’s probably possible to fix the partition so that it’s as big as it used to be. It’s likely that some of your data is corrupted already, but the repartitioning won’t have erased the old data except here or there where it’s written things like new file tables in space it now considers unused