Is it a mistake to use a password that has previously been used (by anyone ever)?

The math may be right. One could refine and complicate it as much as desired, but it doesn't really add to the point. So I'll leave it be.

Also, in practice it is easier—and might be faster—to check for any random character password with a fixed length than to check unique passwords from a list. A password list with 243 passwords with an average password length of 8 characters would be about 64 TB in size, if my calculation is correct. This would have to be stored somewhere in close proximity to the processor to be read with the same speed as the processor calculates the hashes.

The conclusion however, is not right: The important question is not if a password has ever been used, but if the password has ever been included in a breach.

If the breached passwords were thereafter publicly disclosed, they are now available on the internet. The passwords are now not just any passwords that have been used, but a very small subset of them. And to make things worse, this subset is used in wordlists by a lot of people around the world to check if they have been reused. So the chance that someone checks a hash against this password is a lot higher than the chance of him or her checking a hash against an unknown password, even if it has been used somewhere.

So I would not use a password that is included in the HIBP database, simply for the reason that those passwords have a higher chance to be included in wordlists.

Mike Ounsworth here (author of the thread you're referencing)

This is a great excuse to do some back-of-the-envelope math! The factor to think about here is that when you're getting to numbers like 243, you have to start factoring in the number of hard drives, CPUs, and electricity required to store and use that data.

To make math easy, let's say each of those 243 password is stored as a SHA-1 hash (as is the case with the HIBP database). Each SHA-1 value is 160 bits, or 20 bytes. 243 * 20 bytes = 176 terabytes. Larger than my laptop, but chump change for a cloud service.

Going the other direction, imagine you have a database of all 243 plaintext passwords. You get your hands on the hash of an admin's password and you want to brute-force it against your database. Let's take the simplest and most insecure case; it's an unsalted SHA-256 hash. This is the problem that bitcoin mining rigs were built for baby! Let's take this bitcoin miner as a rough benchmark: $3,000 USD, 50TH/s (tera-hash per second), and consumes 1975 W.

According to my hasty math, one of those units would take 2^43 / (50,000,000,000,000 / s) = 0.2s to try all passwords, assuming that a database can feed 176 TB of data to it that quickly.

In reality, passwords are (well, should be) stored with salted PBKDF2 or Argon2. This changes the game considerably as these hash functions are intended to prevent this kind of attack. These hash functions can be tuned as slow as you want at the time that you store the password as a hash. Say you tune it to be ~ 0.1 s per hash. Now suddenly you're looking at numbers like "thousands of centuries", and "power consumption of the planet".

TL;DR: this is a great question to ask!

The answer is that if you're going to choose a password that you can remember and might collide with someone else on the internet, then your choice of password is less important than the site you're giving it to storing it securely.

IMHO, in choosing a password, you're not trying to prevent a dedicated enough attacker from ever cracking it; instead you're trying to make it hard enough that they'll go after a softer target. "I don't need to outrun a bear, I just need to outrun my friend".

If course, if you use a password manager with a completely random 32-char password, then you're getting into the cryptographic strength realm of "age of the universe", and "power output of a large star". So do that!

I see some logical errors with that statement - first of all, how would you ever know it?
If Joe Schmoe used a specific password in 2007 - 2009 for his Windows PC, and it was never hacked, and the machine is trashed and burned, there would be no record of it anywhere.
Therefore, unless a password was hacked or published in any other way, you cannot know, and so cannot avoid reusing it.

Aside from that, of the estimated 2^43 passwords ever used, probably 2^42.9 are duplicates, and the list fits on one hard disk.