Heck if I know. Whatever the consequences of flashing a custom recovery are, maybe?
I think the risks are pretty low...but I guess it depends on how comfortable you are with what you are doing. And maybe what understanding you have of what's going on...(someone else correct me here if I am off base).
You download a stock nandroid backup in step 15. Step 16 has you unzip it to the root of the SD card. The zip of the backup has a directory structure in it, so it will unzip a bunch of directories, (some of which, like nandroid, probably already exist on your SD card). The directories will merge.
Typing "su" in the terminal gives you superuser access. I am not sure if this is required to run the flash_image script or not...
You then cd (change directory), into the nandroid backup directory, where you run the flash_image script telling it to overwrite the current recovery with the recovery.img in the nandroid backup directory.
Should work fine, but again, never tried it.
I guess with anything, "worst case" is bricking the phone.
More importantly, WHY are you wanting to flash the stock image???