It starts with a bold claim: "ssh accounts that should not accept a password will accept any password between 256 and 1024 characters". But then it keeps adding qualifiers:
- sshd compiled with musl
- not using PAM
- password logins enabled
- at least one account with '*' as the password "hash"
- at least one OTHER account with a valid password
- server is aligned with 2 ley lines
- hacker is wearing a blue t-shirt and green socks
Ok, the last two are an exaggeration, but still. The conditions are all ANDed, miss a single one and you're fine.
On top of that, as someone else in these comments reminded, the target account needs to have a valid login shell or other exploitable program assigned to it, and these passwordless accounts tend to have nologin or something similar.
To me this seems to fall on the "technically possible, practically very unlikely" bucket, kinda like the PHP vulnerability from the other day. Maybe this has gone unnoticed for so long because it just doesn't happen.