As the tweet points out the bad practice and doesn't comment on the good practice how _should_ one store their keys? Specifically the tweet states that using a secret manager or password vault is a problem, so what is the solution?
But if you're an exchange handling billions of dollars of customer assets, the requirements should probably be higher. The text implies that many employees at the company had access to the password vault, for example. Also, shared password vaults that I've seen tend to have functionality like the ability to share a password externally (something you probably don't want!), relatively low logging abilities (while it would probably be a good idea to track each and every time a crypto key was accessed and who acccesed it), etc.
At least that's my guess at what they meant — perhaps someone had deeper knowledge and can share that.
Analogy in a old bank with cash or gold is hot wallet = cash that tellers have on hand, cold wallet = vault in the back that has everything else.