That was a timing weakness in an older version of iOS that was patched in software. Previously you could just cut power at a precise monent and get unlimited attempts. It's no longer exploitable.

Does that mean that now if you cut power at the same moment the fail counter increments even if the entered code was correct?

Not that it's particularly useful for hackers, I'm just wondering if this can be done "perfectly" at all.

Yes you can, if you sync the commit to NVRAM before giving any external indication of success/failure, and don't leak through any side-channels. The CVE before demo'd by the famous youtube video was that you had a split second after failure was indicated where you could cut the power and keep the failure from being stored.