if anyone is still trying to get this to work, use StarDust Toolkit.
https://github.com/StarDustCFW/StarDust-Toolkit/releases
it is the original creator of LinkUser (which is what kefir uses from what i've read in this thread)
i just tried this on my 10.0.2 emuNAND and it does it seamlessly. i didn't even have to restore my old saves.
edit: here is the link to LinkUser in case anyone wants to verify
https://github.com/StarDustCFW/LinkUser
edit2: the reason i wanted to do this was to launch SNES Online. it allows me to launch it now instead of asking me to link account. so i can verify it works for that too.
edit3: after using this for a couple weeks, i was experiencing major battery drain on standby (like ~30-50% when i sleep) but it was only when i was connected to wifi. after lots of troubleshooting, i found out it was due to the fake linked account. once i unlinked the account, i don't have any battery drain (usually only ~1-2% drain now)
if you are experiencing battery drain, either turn on airplane mode or unlink the account through goldleaf.
either this has something malicious in the background or it's due to the blank dummy account trying to get the account information from nintendo. i was using 90DNS so it was constantly trying to connect but getting blocked. since there really isnt any important information on the switch i'm guessing it's the latter.