Agreed. But since every character gives you around 6 bits (26*2 letters + 10 numbers + some special characters ≈ 64 = 2^6), you'd need 256/6 ≈ 43 characters to exhaust the checked entropy, so up to that level it makes sense.
If you use sentences instead of randomly generated characters, the entropy (in bits/character) is lower, so 100 characters might well make sense.
Passwords are (or, rather, SHOULD be) cryptographically hashed rather than encrypted. It's possible to compute a hash over data which is longer than the hash input block size by feeding precious hashes and the next input block back in to progressively build up a hash of the entire data.
bcrypt, one of the more popular password hashing algorithms out there, allows the password to be up to 72 characters in length. Any characters beyond that 72 limit are ignored and the password is silently truncated (!!!). It's actually a good method of testing whether a site uses bcrypt or not. If you set a password longer than 72 characters, but can sign in using just the 72 characters of your password, they're in all likelihood using bcrypt.
It's not broken. It's just potentially less helpful when it comes to protecting poor guessable passwords. bcrypt isn't the problem, weak password policies/habits are. Like bcrypt, argon2 is just a bandaid, though a tiny bit thicker. It won't save you from absurdly short passwords or silly "correct horse battery staple" advice, and it's no better than bcrypt at protecting proper unguessable passwords.
Also, only developers who have no idea know what they're doing will feed plain-text passwords to their hasher. You should be peppering and pre-digesting the passwords, and at that point bcrypt's 72 character input limit doesn't matter.
Bcrypt alone is unfit for purpose. Argon2 does not need its input to be predigested.
It's easy for somebody who knows this to fix bcrypt, but silently truncating the input was an unforced error. The fact that it looks like and was often sold as the right tool for the job but isn't has led to real-world vulnerabilities.
It's a classic example of crypto people not anticipating how things actually get used.
Peppering is for protecting self-contained password hashes in case they leak. It's a secondary salt meant to be situated 1) external to the hash, and 2) external to the storage component the hashes reside in (i.e. not in the database you store accounts and hashes in). The method has nothing to do with trying to fix anything with bcrypt. You should be peppering your input even if you use Argon2.
Using memorable passphrases online is always a bad option because they're easily broken with a dictionary attack, unless you bump the number of words to the point where it becomes hard to remember the phrase. Use long strings of random characters instead, and contain the use of passphrases to unlocking your password manager.
If you use sentences instead of randomly generated characters, the entropy (in bits/character) is lower, so 100 characters might well make sense.